ELEMENTS 
OF AVIATION ENGINES 



ELEMENTS OF 

AVIATION 
ENGINES 

By JOHN B. F. BACON, PH. B. 

Instructor, Engines Department 

U. S. School 

of Military Aeronautics 

Berkeley, California 



PAUL ELDER AND COMPANY 

SAN FRANCISCO M CM XVIII 



d 



COPYRIGHT, 1918, BY 

JOHN B. F. BACON 
BERKELEY, CAL. 



^503535 

SEP 



g-ni)*) 



CONTENTS 

PAGE 

Introduction VII 

CHAPTER I 

The Aviation Engine 3 

CHAPTER II 

Application of the Basic Principle .... 7 

CHAPTER III 

Engine Specifications 16 

CHAPTER IV 

Engine Parts 22 

CHAPTER V 

Carburetion 47 

CHAPTER VI 
Ignition 57 

CHAPTER VII 

Lubrication 71 

CHAPTER VIII 
Cooling 80 

CHAPTER IX 

Rotary Engines 84 

CHAPTER X 

The Liberty Motor 96 

Index 105 



III 



ILLUSTRATIONS 

facing page 
Thrust Bearings [36 

Diagram to Illustrate the Curtiss Ox Valve 
Action 42 

The Miller Aviation Carburetor . . .50 

A Half Section View of a Zenith Carburetor . 52 l 

Diagrams to Illustrate the Location of the Core 
in a Shuttle Type Magneto 58 

Wiring Diagram of a Magneto System . . 62 - 

Diagram to Illustrate the Principle of Revolv- 
ing Poles on the Dixie Magneto .... 64 

Diagram to Illustrate Position of Rotor in the 

Dixie Magneto when the Core is Magnetized 66 ' 

Diagram to Illustrate Position of Rotor in the 
Dixie Magneto when the Core is Demagne- 
tized .66 

Diagram of a Battery System of Ignition with a 
Non Vibrating Coil 68 

Gear Pump 76 

Diagram to Illustrate the Operation of a Vane 
Pump .... 76 

Centrifugal Pump 82 

Diagram to Illustrate the Principle of a 

Rotary Engine 84 1/ 



[V 



INTRODUCTION 

Having been forcibly' impressed with the 
fact that many of those who take up the 
study of aviation are not familiar with gasoline 
engines and have little mechanical inclination, 
it has been the endeavor of the writer to explain 
in a simple way some of the points that appear 
to cause beginners the greatest amount of trouble. 
While it may aid those who are conscientiously 
reviewing the subject, it is far from the purpose 
of this book to provide a short cut to passing 
marks on examination papers. 

All of the information herein contained has 
been before the engineering public at one time or 
another. Realizing that certain new develop- 
ments must not appear in print during this 
critical period every precaution has been taken 
to observe strict avoidance of revealing confiden- 
tial information. 

The writer wishes to express his gratitude to 
the members of the Engines Department in the 
S. M. A. of Berkeley for their assistance. Special 
thanks is due Mr. James Irvine for his sugges- 
tions which have resulted in many improvements. 

JOHN B.F.BACON, 
818th Aero Depot Squadron, U. S. A. 



Berkeley, Cal., August, 1918. 

[VII] 



ELEMENTS 

OF 

AVIATION ENGINES 



ELEMENTS 
OF AVIATION ENGINES 

CHAPTER I 
THE AVIATION ENGINE 

In taking up a new subject it is often best 
to fix clearly in mind just what is meant by 
the name of the subject, so in beginning a dis- 
cussion upon aviation engines it seems well to 
start with a rough definition of the term avia- 
tion engine. A simple statement that an in- 
ternal combustion engine so designed that it is 
capable of lifting from the ground and sustain- 
ing in flight a heavier than air flying machine 
will suffice as a definition for our subject. By 
the term internal combustion engine is com- 
monly meant simply a gasoline engine, because 
in such an engine the power is derived from the 
force of an explosion within a cylinder. This 
will make clear what we mean by our subject. 
The question at once arises : Why must avia- 
tion engines be internal combustion engines in- 
stead of steam engines, and why not propel 
aeroplanes by aid of electricity? The answer is 
simply that maximum power and minimum 

[3] 



ELEMENTS OF AVIATION ENGINES 

weight can be best obtained with the internal 
combustion engine. In the study of aeronau- 
tics weight is a tremendous factor, and it is in- 
teresting to note that not until the gasoline 
engine had reached its modern development 
was human flight practical. On account of the 
unlimited use of gasoline as a motive power 
and the increasing interest of technical men in 
the problems of aviation, the gasoline engine 
has been developed to such a point that it may 
deliver 1 H.P. for every 1.8 pounds of its 
weight. To a mechanical mind this seems one 
of the greatest achievements of the twentieth 
century. 

Since gasoline engines have been used so ex- 
tensively and with such marked success in 
automobiles, the aviation student will at once 
involuntarily compare the aviation engine with 
that in an automobile, and oftentimes he com- 
pares them wrongly by stating that the avia- 
tion engine develops a vastly greater speed 
than the engine of an automobile is capable of 
attaining. This is incorrect and is a poor way 
of comparing the two. The main difference is 
that of lightness. Aviation engines are of the 
lightest possible construction and are designed 
to run continuously at their highest speed. 

[4] 



THE AVIATION ENGINE 

Seldom are the frail supporting members for 
the engines in a horizontal plane, and often the 
engine is called upon to do its work while com- 
pletely inverted. These are conditions that the 
automobile engine does not have to meet. In 
order to attain a construction that will fulfill 
the requirements imposed upon aviation en- 
gines, it is natural to expect that some sacri- 
fice must be made. This accounts for their low 
degree of durability. When we examine the 
heavy construction of a 400 H.P. marine gaso- 
line engine and then regard the frail parts of a 
400 H.P. aviation engine there is not the 
slightest doubt which engine will continue 
longer in its operations. However, since light 
construction is an absolute necessity, it is use- 
less to expect much in the way of durability, 
and as a means of knowing what an aviation 
engine will stand it is interesting to note that 
after every seventy-five hours of operation the 
engine should be rebuilt. 

As a compact and light power plant the avia- 
tion engine is the highest attainment of me- 
chanical genius. It has been developed from 
the type that propels the automobiles, and 
just as the old types of automobile engines do 
not resemble in appearance the types used to- 

[5] 



ELEMENTS OF AVIATION ENGINES 

day, so the first aviation engines have little 
resemblance to those of the present time. The 
development has been rapid, and it is difficult 
to predict what will be the effect upon aviation 
if the rapid strides taken during the past ten 
years continue to add to the efficiency and re- 
liability of the aviation engine during the next 
ten years to come. 



[6] 



CHAPTER II 

APPLICATION OF THE BASIC 

PRINCIPLE 

The working principle of an aviation en- 
gine is identically the same as that of the 
ordinary gasoline engine. In the middle of the 
nineteenth century it was satisfactorily proven 
that the explosive force of gasoline could be 
used to actuate a piston, and this has given 
rise to the adoption of a new form of motive 
power. Since that time gasoline engines have 
been developed along two lines, one being 
called the two-stroke cycle engine, and the 
other the four-stroke cycle engine, but since the 
former has not been used extensively in aviation 
work little attention will be given to it here. 
A two-stroke cycle engine is one in which an 
explosion takes place in the cylinder every 
time the crank shaft makes one revolution. A 
charge of combustible gas is slightly com- 
pressed within the crank case by the piston 
traveling downward. Near the bottom of this 
downward stroke the piston uncovers a port in 
the cylinder wall allowing some of the com- 
pressed gas to enter the cylinder. Then the 

[7] 



ELEMENTS OF AVIATION ENGINES 

piston moves upward, closing the port and 
compressing the gas. The charge is ignited 
when the piston is near the end of its upward 
stroke, and the result is that the force of the 
explosion violently drives the piston down- 
ward. An exhaust port on the opposite side of 
the cylinder from the intake port is uncovered 
as the piston sweeps downward, and the force 
of the explosion starts the burnt gas rushing 
out of the cylinder. The intake port having 
also been uncovered by this time will allow a 
fresh charge to enter. By using a deflector on 
the piston head the fresh charge is hindered 
from rushing straight to the exhaust port and 
is diverted upward, serving admirably to expel 
the remaining burnt gases. Now the piston is 
ready to go upward again, and the same opera- 
tions are repeated. In this way the piston 
makes two strokes to complete a cycle, hence 
it is spoken of as the two-stroke cycle engine. 

Some confusion may be caused by not 
knowing the exact meaning of the word cycle, 
so it may be well to insert here a definition. A 
complete series of events occurring in regular 
sequence and ending so that the same opera- 
tion can be repeated in the same order is called 
a cycle. 

[8] 



APPLICATION OF THE BASIC PRINCIPLE 

The four-stroke cycle engine has proven the 
more satisfactory of the two types, and since 
it is the one used in connection with aviation, 
it is very desirable to fully understand it. This 
type differs from the two-stroke cycle in that 
it has two distinct mechanically-operated 
valves in the cylinder which, of course, necessi- 
tate a few more working parts. Instead of the 
gas being stored and compressed within the 
crank case, this engine draws its explosive 
charge directly from the carburetor by opening 
the inlet valve as the piston goes downward 
and making use of the suction thus exerted. 
The charge is compressed by the reversal of the 
piston's motion and the closing of the inlet 
valve. Near the end of this compression stroke 
the charge is ignited, resulting in an explosive 
force being exerted on the piston when it is 
ready to go downward again. Near the end of 
this succeeding downward stroke the exhaust 
valve is opened permitting the force of the ex- 
plosion to give the burnt gases their initial 
outward impulse. The valve remains open 
during the entire upward stroke of the piston 
to insure all of the burnt gases being expelled. 
The clearing out of the cylinder is often re- 
ferred to as scavaging the cylinder. Generally 

[91 



ELEMENTS OF AVIATION ENGINES 

the exhaust valve closes after the piston has 
reached its uppermost position. This brings us 
to the opening of the inlet valve and with that 
the sequence of events is repeated. 

By the stroke of the piston is meant the 
movement of the piston in one direction. It 
follows from this that the length of the stroke 
is the linear distance the piston travels from 
its uppermost position to its lowest position or 
vice versa. The term stroke has come to mean 
simply the number of inches between top cen- 
ter and bottom center, thus designating the 
two extreme positions of the piston. To make 
clear the four strokes of the piston in a four- 
stroke cycle engine, the first one in which the 
piston goes down and draws in a charge is 
called the intake stroke. The next upward 
motion is the compression stroke. Then comes 
the explosion which drives the piston down- 
ward. This is the power stroke. Finally the 
expulsion of the burnt gases is the exhaust 
stroke, and this completes the cycle. 

In aviation engines it is customary to ignite 
the charge near the end of the compression 
stroke instead of at the beginning of the power 
stroke. The speed of the engine justifies this. 
If ignition were to take place when the piston 

[10] 



APPLICATION OF THE BASIC PRINCIPLE 

was at top center or a little afterward, the 
force of the explosion would be exerted upon 
the piston head at such a late time that the 
piston could not deliver its maximum impulse 
to the crank shaft. When the piston is nearing 
bottom center its effectiveness for transmitting 
force is negligible. Consequently by opening 
the exhaust valve at the end of the power 
stroke instead of at the beginning of the ex- 
haust stroke, the force of the explosion serves to 
start the burnt gases rushing outward without 
losing power. The exhaust valve is generally 
held open until the beginning of the intake 
stroke. This aids in scavaging the cylinder as 
it permits more time for the operation, and the 
danger of retaining some of the burnt gases 
is avoided since the out-going exhaust will 
possess a certain amount of inertia. Different 
makes of engines have different times for open- 
ing the intake valve. On some there is a 
small interval between the closing of the 
exhaust valve and the opening of the inlet 
valve, as is the case with the Curtiss OX and 
the Hall-Scott. This permits the downward 
motion of the piston to establish somewhat of 
a rarefication within the cylinder, so that when 
the inlet valve is opened there will be a ten- 

[111 



ELEMENTS OF AVIATION ENGINES 

dency for the gas to enter more promptly. 
The closing of the inlet valve occurs at the be- 
ginning of the compression stroke. The gas 
passing through the manifold will have some 
inertia which will maintain a flow into the 
cylinder during the first part of ensuing up- 
ward stroke. By thus keeping the valve open 
past bottom center a larger amount of gas is 
placed in the cylinder. 

The question often arises : Why are not two- 
stroke cycle engines used for aviation work, on 
account of the decrease in weight due to the 
less number of working parts, the more fre- 
quent power impulses, and the need of an 
engine that will do its best work when running 
at top speed? The two-stroke cycle engine ful- 
fills all the requirements demanded of an 
aviation engine except for the fact it will not 
ordinarily run satisfactorily at low enough 
speeds to allow the propeller to idle. Since a 
successful aviation engine must be able to run 
slow enough without stopping to allow the 
plane to glide, it can be easily seen that the 
present form of two-stroke cycle engine is 
poorly suited for aviation work. 

So far in explaining the different operations 
involved in a cycle, only one cylinder has been 

U2 1 



APPLICATION OF THE BASIC PRINCIPLE 

considered. It is advisable to have frequent 
power impulses and to avoid vibration as 
much as possible. This is accomplished by 
using a number of cylinders which decreases 
the weight of the reciprocating parts. 

Vibration is due to the shifting of the cen- 
ters of gravity of pistons and connecting rods. 
In a single cylinder engine of required power 
turning at a speed suitable to drive a propeller, 
the amount of vibration would be prohibitive. 
The greatest bearing pressure in an engine at 
high speeds comes not so much from the ex- 
plosion, but from the effort of starting and 
stopping the weight of the piston and connect- 
ing rod. To decrease this reciprocating weight 
it is necessary to resort to the basic law of 
volumes and areas. If we make a body half the 
dimensions of another, it will have but one 
quarter of the area and only one-eighth of the 
weight. This can be applied to pistons. Thus a 
piston can be replaced by four smaller ones half 
as large, and the area of the four will equal that 
of the larger one. However, these four pistons 
will weigh practically one-half as much as the 
original single piston. This illustrates the way 
reciprocating weight is lessened and shows plain- 
ly the demand for a larger number of cylinders. 

[13] 



ELEMENTS OF AVIATION ENGINES 

The way the cylinders are arranged serves as 
a means of classifying aviation engines. If the 
cylinders stay in a fixed position in respect to 
the crank shaft, it is spoken of as a fixed cylin- 
der engine, but if the cylinders revolve about 
the crank shaft it is called a rotary engine. 
Various difficulties in construction are encoun- 
tered when the number of cylinders is increased, 
so fixed-cylinder engines are not confined to 
the vertical style but are often built in a V 
form to permit a shorter crank shaft. A pecu- 
liar style of fixed-cylinder engine is that with 
an additional row of cylinders between the 
two rows that go to make up the V. This is the 
design of the Sunbeam Engine. Another style 
of fixed-cylinder engine is one in which the 
cylinders radiate from the crank case allowing 
the force of all explosions to be exerted upon 
the same crank pin. The Anzani engine is of 
this design. The rotary engines have not so 
many variations. As a means of increasing the 
number of cylinders a second bank of cylinders 
is often added, which of course necessitates 
two throws on the crank shaft. Rotary engines 
are limited to those having one and two banks. 
In both the fixed-cylinder and the rotary types 
the growing demand for an increased number 

[14] 



APPLICATION OF THE BASIC PRINCIPLE 
of cylinders has resulted in the adoption of en- 
gines of the designs just referred to for aviation 
work. 



[15] 



CHAPTER III 
ENGINE SPECIFICATIONS 

As a basis of comparing aviation engines 
_t\. certain specifications are used. It must be 
remembered that all engines are not called up- 
on to do the same work, and furthermore that 
they are not all designed by one man or even 
by a group of men holding the same views on 
various mechanical problems. This will ac- 
count for the wide range in specifications. In 
order to become familiar with the points where 
engines differ, a few items will be taken up here. 

The first point to consider is whether the 
engine has fixed cylinders or is a rotary. If it 
is a fixed-cylinder engine, the arrangement of 
the cylinders should be noted. Generally 
speaking, rotary engines are used for very fast 
but brief flights, while fixed-cylinder engines 
serve better for long flights where speed is not 
so important. 

The horse-power of an engine is probably the 
matter of greatest interest. All planes are not 
of the same size and weight, so there is need 
for engines of different power. One horse- 
power is the power required to lift 33,000 

[161 



ENGINE SPECIFICATION 

pounds a distance of one foot in one minute. 
The horse-power necessary to operate a plane 
is calculated by multiplying the total air re- 
sistance of the plane, expressed in pounds, by 
the speed in feet per second, then by 60 sec- 
onds in a minute, and dividing the product by 
33,000. The actual horse-power that an engine 
develops is spoken of as brake horse-power. 
It may be found by measuring the torque 
exerted by the engine running with a propeller 
attached. By torque is meant the moment of 
tangential effort, or to put it more roughly, a 
force tending to produce rotation. The torque 
is allowed to be exerted upon an arm which 
delivers the force to a platform balance. By 
multiplying the force in pounds by the dis- 
tance in feet through which it acts in one revo- 
lution by the R.P.M. and dividing the product 
by 33,000, the actual horse-power is obtained. 
The distance through which the force acts is 
the circumference of a circle having the power 
arm as a radius. This distance will be 6.2832 
times the arm's length, so if we make the arm 
exactly 534 feet long, the distance through 
which the force acts will be 33 feet. This per- 
mits us to reduce our fraction to the lowest 
terms, making the denominator 1,000 instead 

[17] 



ELEMENTS OF AVIATION ENGINES 

of 33,000. The horse- power can then be ob- 
tained by multiplying the torque expressed in 
pounds by the R.P.M. and then dividing by 
1,000, which simply amounts to moving the 
decimal point three places to the left. 

The weight of an engine is of great import- 
ance, for it determines the engine's fitness. As 
has been said before, aviation work requires 
maximum power for minimum weight. Light- 
ness is the keynote of the whole engine, so the 
aviation engine is devoid of all unnecessary 
equipment. Self-starters are seldom used on 
account of their weight and mufflers never, on 
account of their weight and resistance also. 
Aviation engines avoid the use of a fly-wheel, 
on account of the large number of cylinders 
and also on account of the steadying effect of 
the propeller. In speaking of the weight of an 
engine, the weights of tanks and radiators are 
not included, nor does oil or water enter into 
the engine's weight. By dividing the weight 
by the horse-power the weight per horse-power 
is obtained. This is a very significant figure 
and is widely used in comparing engines. The 
most modern types of aviation engines range 
from two to three pounds in weight for every 
horse-power developed. 

[18] 



ENGINE SPECIFICATION 

The speed of most aviation engines is gener- 
ally about 1,400 R.P.M. being a compromise 
between the most efficient propeller speed and 
the most efficient engine speed. An ordinary 
propeller will do its best work when turning 
from 900 to 1,000 R.P.M. If it is driven con- 
siderably faster than that, it will cause what 
is known as cavitation, which means that the 
blades are working in an unfavorable medium 
so far as their usefulness is concerned. This 
will show the undesirability of having pro- 
pellers turn at speeds which a high-grade auto- 
mobile motor can easily attain. Consequently 
since the speed of an engine is normally 
greater than 900 or 1,000 R.P.M. it is advis- 
able to compromise by driving the propeller a 
little faster than it ought to turn and running 
the engine at a reduced speed. The efficiency 
of an engine, which roughly speaking is the 
proportion between the energy received as 
work and the energy supplied as fuel, can be 
increased if the engine is permitted to run 
faster than 1,400 R.P.M. Since the propeller 
speed has limitations, engines running at 
higher speeds must have a gear reduction re- 
garding the propeller. This is ordinarily ac- 
complished by driving a jack shaft carrying 

[19] 



ELEMENTS OF AVIATION ENGINES 

the propeller by spur gears one above the 
other. Sometimes internal gears are used and 
then the propeller will turn in the same direc- 
tion that the engine turns. 

The disadvantages of a geared propeller are 
that more weight is added and a slight amount 
of power is consumed by the gears. 

The direction of rotation of an engine should 
be considered. When standing directly in front 
of the propeller and noting that it turns coun- 
ter-clockwise, the engine is spoken of as having 
a normal rotation. Should the propeller turn 
clockwise the engine has an anti-normal rota- 
tion. One reason for building both normal and 
anti-normal engines is that in case a plane has 
two engines as is sometimes the case with 
bombing planes, then normal and anti-normal 
engines are used to equalize the torque effect. 

The number of cylinders and their bore, 
meaning the internal diameter, is an important 
item. The stroke of the piston which has been 
mentioned before is often spoken of in connec- 
tion with the bore. Various engines use differ- 
ent strokes with different bores, but for the 
sake of illustration, the stroke averages about 
one and one-quarter times the bore. If both 
the bore and the stroke are large, there will be 

[20] 



ENGINE SPECIFICATION 

a tendency to develop heat on the compression 
stroke providing the compression chamber is 
small. The total piston displacement is calcu- 
lated by squaring half the bore, multiplying by 
3.1416, then multiplying by the stroke, and 
finally by the number of cylinders. The result 
will be in cubic inches. The horse-power per 
cubic inch of piston displacement, which is 
obtained by dividing the horse-power by the 
displacement, is a figure of much interest. 
Efficient motors will give from .17 to .27 H.P. 
for each cubic inch of displacement. 

Ignition, carburetion, and cooling enter into 
the specifications of an engine, but since separ- 
ate chapters are devoted to them later, they 
need not be dealt with here. 



[21] 



CHAPTER IV 
ENGINE PARTS 

To take up all the parts of an engine and 
describe them fully would be a big under- 
taking, and might not prove interesting to 
those beginning this subject. Consequently 
only the principal parts will be included and 
dealt with in a very brief manner. 

The cylinders of a gasoline engine are vari- 
ously constructed. They may be made as in- 
dividual units, or several may be cast in block. 
The advantage of the former method of con- 
struction is that more complete jacketing can 
be accomplished, while rigidity is the advan- 
tage of the latter type. In case an engine had 
four cylinders cast in block and one became 
damaged, then the three good ones would have 
to be discarded in order to replace the one 
cylinder that caused the trouble. This waste 
is not encountered when each cylinder is a 
separate replaceable unit. However, from the 
standpoint of compactness the block construc- 
tion is by far the more preferable. Individual 
cylinders are made of cast iron, semi-steel, and 
steel. When cast in block their material is 

[22] 



ENGINE PARTS 

usually aluminum alloy. A peculiar form of 
construction is that used in the Curtiss cylin- 
ders, where each cylinder is of cast iron with a 
band of some non-corrosive metal such as 
monel metal to act as a water jacket. The 
cylinders of the Hispano-Suiza are unusual in 
design, being steel thimbles that screw into an 
aluminum alloy water jacket designed to hold 
four cylinders. The Sturtevant cylinders are 
interesting in that they are of aluminum alloy 
cast in pairs with a steel liner shrunk in to act 
as a cylinder wall. 

The location of the valves determines the 
shape of the cylinder head. If the valves oper- 
ate in extensions on opposite sides of the com- 
bustion chamber the cylinder is said to have a 
T head, since its shape is that of a T. This con- 
struction necessitates two independent cam 
shafts besides being rather bulky, so is of little 
importance from the standpoint of aviation 
work. If a cylinder has only one extension in 
which a valve or valves work, its shape will 
resemble that of a Greek letter gamma or sim- 
ply an inverted L. It is therefore called an L 
head. When a cylinder has no extensions on 
either side but has two valves located in its 
head, it is called an I head cylinder. This type 

[23] 



ELEMENTS OF AVIATION ENGINES 

of cylinder is the most populai for aviation 
engines, because it does away with an irregu- 
larly-shaped combustion chamber. In the case 
of a T or L head cylinder the space above the 
valves may be regarded as a pocket, and very 
often it is difficult to scavage these pockets. 
The placing of both valves in the head permits 
the combustion chamber to be made slightly 
spherical in order to reduce the surface area 
and lessen the amount of heat carried away at 
the time when an explosion takes place. 

Some cylinders are made so that the head 
may be removed without disturbing its base. 
This is known as a detachable head and has 
the advantage of providing an easy means of 
removing carbon and working upon the valves. 
However, a little more material is required in 
this construction, and it brings into account 
compression leaks and also water leaks since 
the cylinder heads must be jacketed. 

The crank case is generally divided into two 
parts ; the top section serving as a base for the 
cylinders and the bottom section carrying a 
supply of oil. The sump is that part which 
holds the oil. As a rule crank cases are alumi- 
num castings, and in case the motor is a V type 
great care is taken to strengthen the upper sec- 

[24] 



ENGINE PARTS 

tion by means of partitions or webs to prevent 
the strain exerted by explosions on opposite 
banks from cracking the upper section. The 
crank shaft bearings are generally held in the 
upper section. Sometimes the lower halves of 
the bearings are held in partitions in the lower 
section of the crank case, as in the Hispano- 
Suiza. The difficulty in this construction is 
that the lower section can not be removed 
without disturbing the crank shaft. As a 
means of retaining the oil in the sump when 
the engine is momentarily inverted, splash 
pans are placed in the lower section. They do 
not retain all of the oil, but aid in reducing the 
amount that would otherwise rush into the 
cavities of the pistons. The vents on crank 
cases are called breathers. These maintain 
atmospheric pressure in the crank case even 
though compression leaks are present. 

That ■ part of the engine which is driven 
downward within the cylinder by the force of 
an explosion is the piston. Pistons have re- 
ceived as much if not more attention by de- 
signers than any other part of the engine, and 
the result has been to secure satisfactory oper- 
ation at high speeds and at high temperatures. 
The material used in piston construction is 

[251 



ELEMENTS OF AVIATION ENGINES 

generally aluminum alloy, although cast iron 
is sometimes used. The use of aluminum as 
piston material serves to lessen vibration and 
increase the speed, lessening the weight of re- 
ciprocating parts. Another reason for its use 
is the rapidity with which it conducts heat. 
The piston head may be either convex, flat, or 
concave, and all of these shapes are in use at 
present. The convex or domehead brings into 
account the ability of an arch to withstand 
strain. Greater strength for a given amount of 
material is obtained by using a convex head. 
The flat head is the common type. By having a 
flat surface less area of the piston is exposed to 
absorb heat. This results in a slightly cooler pis- 
ton, which is a big advantage, as it is impossible 
to cool the piston in the same way that the cyl- 
inder is cooled. The concave head has been ex- 
tensively used on rotary engines because it 
permits a shorter cylinder and thus lessens the 
centrifugal force. This shape of piston head al- 
lows the combustion chamber to assume a spher- 
ical form. By the bosses are meant the two 
projections within the piston that hold the wrist 
pin, and it follows that the upper end of the con- 
necting rod must fit between the two bosses. 
The lower portion of a piston is termed the skirt. 

[26] 



ENGINE PARTS 

Due to more material at the head and also 
on account of the top surface coming in direct 
contact with the heat of each explosion, it will 
be seen that the upper part of the piston will 
expand more than the skirt. This necessitates 
allowing more clearance between the cylinder 
wall and the piston at its head than at its 
skirt. Some idea of this difference can be had 
by pointing out that a five-inch piston may be 
cleared .020 inch at the skirt and as much as 
.027 at the head. 

To prevent compression and the force of an 
explosion from passing down between the pis- 
ton and the cylinder wall, piston rings are 
used. These fit in grooves in the piston and 
bear upon the cylinder wall. Besides prevent- 
ing leaks these rings prevent much oil from 
getting upon the piston head where it would 
result in the formation of carbon. The rings 
are made of cast iron, and each piston gener- 
ally requires two or three of them. When the 
two ends of a ring come together squarely 
the ring is said to have a butt joint. When the 
ends meet each other diagonally it is called a 
diagonal joint. Likewise if the ends are made 
so that they meet each other in the form of a 
step, it is called a step or lap joint. Obviously 

[27] 



ELEMENTS OF AVIATION ENGINES 

a ring having a step joint will offer more resis- 
tance to the passage of gas than those having 
butt or diagonal joints. A precaution to take 
in placing a piston in a cylinder is to make sure 
the joints in the rings are at equal intervals 
around the circumference of the piston. 

The wrist pin is made of steel, usually hol- 
low and case hardened, and is used to form a 
movable joint between the piston and the con- 
necting rod. Its length depends upon the 
diameter of the piston. There are three gen- 
eral ways of retaining the pin in its right posi- 
tion. It may be held rigidly in the connecting 
rod by means of a clamp or a set screw which 
results in the pin turning in the piston bosses 
as the connecting rod moves back and forth. 
This method is used in the Curtiss OX. An- 
other way is to pin the wrist pin in the bosses 
so that it is securely held in a fixed position. 
The connecting rod will then turn on the wrist 
pin which means the bearing will be in the 
connecting rod. Such a construction necessi- 
tates a bearing at both ends of the connecting 
rod. The Hall-Scott A5A has its wrist pins 
held rigidly in the piston bosses. The floating 
method such as used in the Sturtevant allows 
the pin to move either in the bosses or in the 

[281 



ENGINE PARTS 

connecting rod. Brass ends on the pin or caps 
over the ends of the bosses protect the cylinder 
walls. 

The connecting rod is a steel arm used to 
convert the reciprocating motion of the piston 
into the revolving motion of the crank shaft. 
The majority of connecting rods have a cross 
section resembling an I, although H and tubu- 
lar rods are not uncommon. In cases where 
the wrist pin is held in the bosses the upper end 
of the connecting rod is supplied with a bronze 
bushing that acts as a bearing surface. The 
lower bearing, in which works the crank pin, 
is given more attention. Babbitt is employed 
as a bearing metal and is generally backed by 
bronze to take its place should enough heat be 
developed to fuse the babbitt. Lower connect- 
ing rod bearings are made in two pieces to 
permit the crank pin being put in position. 
Between the two halves of the bearing are placed 
strips of metal called shims. These are .001, 
.002 and .005 inch thick and as the bearing 
wears away these can be removed insuring a 
better fit. In a V motor, when the vertical axis 
of opposite cylinders are in the same vertical 
plane, the connecting rods of opposite cylin- 
ders will meet the crank pin at the same point. 

[29] 



ELEMENTS OF AVIATION ENGINES 

This will necessitate the forked or straddled 
construction in which one rod works between 
the fork of another. It makes rather a com- 
plicated and costly bearing, but it is a favorite 
design and is being used extensively. The 
Hispano-Suiza has this type of lower connect- 
ing rod bearings. Another and simpler way is 
to have the cylinders "staggered" by placing 
the cylinders on one bank a little ahead or be- 
hind those on the opposite bank, thereby 
allowing two lower connecting rod bearings to 
work side by side on one crank pin. A wise 
precaution to take in assembling a motor is to 
make sure the lower connecting rod bearing is 
such that it allows the wrist pin to be abso- 
lutely parallel with the crank pin. If it is 
otherwise the piston will not work freely within 
the cylinder. 

The crank shaft is the driving shaft of the 
engine to which the power impulses are trans- 
mitted by the pistons and connecting rods. 
It is needless to say that this is the most im- 
portant moving part of an engine, and for this 
reason it is made with great precision from 
selected pieces of high-grade steel by drop 
forging and subsequent turning. The principal 
parts of a crank shaft are the main bearings, 

[30] 



ENGINE PARTS 
the cheeks, and the pins on which the connect- 
ing rods work. Two cheeks and a pin are 
spoken of as a throw. Thus the number of 
cylinders govern the number of throws, and 
also upon the number of cylinders depends the 
number of degrees between the throws. In a 
vertical motor, if the cylinders are cast separ- 
ately, there is generally a main bearing be- 
tween every two throws. Where the cylinders 
are cast in block there is not so much space 
between the pistons which often means a de- 
crease in the number of main bearings. The 
crank shaft used in a V motor is identically the 
same as one used in a vertical motor having 
half the number of cylinders. Two connecting 
rods are fitted to each throw, and if the cylin- 
ders are cast separately a main bearing is 
placed between every two throws. 

For the main bearings of a crank shaft the 
lining is babbitt usually backed by bronze very 
similar to the lower connecting rod bearings. 
Babbitt, which essentially consists of lead and 
antimony, is used as bearing metal because of its 
anti-friction properties, its sufficient hardness, 
and the ease with which it can be replaced. 
Lead alone possesses considerable anti-friction 
properties, but is impracticable on account 

[31] 



ELEMENTS OF AVIATION ENGINES 

of its softness. The addition of some anti- 
mony will materially harden the lead with- 
out lessening its anti-friction properties. The 
use of babbitt also permits the liner to be 
scraped to secure an exact bearing surface. 
By coating the journal with Prussian blue, the 
high spots can be detected on the liner, and 
these can be successively removed by scraping. 

To have evenly placed power impulses the 
throws on a crank shaft must be placed at cer- 
tain angles with one another. In any four- 
stroke cycle motor all cylinders will fire once 
in two revolutions of the crank shaft or once 
in 720 degrees. In a four-cylinder motor there 
would be four explosions in 720 degrees, and to 
get equal spacing the power impulses would 
have to come one-fourth of 720 or 180 degrees 
apart. This will explain why the angle be- 
tween two throws that receive impulses, one 
directly after the other, is 180 degrees for a 
four-cylinder crank shaft. The throws in a six- 
cylinder crank shaft are 120 degrees apart, 
since there will be six power impulses in 720 
degrees. 

In determining the order in which the cylin- 
ders will deliver their power impulses to the 
crank shaft, it is the custom to fire them so 

[32] 



ENGINE PARTS 

that the vibrations set up by one explosion will 
serve to counteract the vibrations caused by a 
previous explosion. To accomplish this an ex- 
plosion at one end of the shaft is followed by 
an explosion near the other end. 

Here we come to what is known as the firing 
order, which simply means the order in which 
the cylinders do their work. In order to discuss 
the firing order it is first necessary to explain 
how the cylinders are numbered. In American 
practice cylinder No. 1 is always that one at 
the pilot's end of the engine, and the number- 
ing is in regular order toward the propeller. 
In V engines No. 1 is the first cylinder on the 
left bank viewed from the pilot's cock pit. 
Some engines have the left bank numbered 
LI, L2, L3, L4, and the right bank Rl, R2, 
R3, R4. Others number the left bank 1, 2, 3, 4 
in the regular way and then start with the 
cylinder nearest the propeller on the right 
bank calling it 1' followed by 2', 3' and 4' 
going toward the pilot's end. The Curtiss OX 
has the peculiar way of starting with No. 1 on 
the left bank nearest the cock pit and desig- 
nating as No. 2 the opposite cylinder on the 
right bank. No. 3 is the next one on the left 
bank, and in this way the odd numbers are on 

[331 



ELEMENTS OF AVIATION ENGINES 

the left bank and the even numbers on the 
right bank. 

To return to firing orders, it is best to start 
with a four-cylinder engine. The cylinders in 
such an engine can be fired in a 1, 2, 4, 3 order 
or in a 1, 3, 4, 2 order. From this it can be seen 
that throws 1 and 2 are 180 degrees apart and 
3 and 4 are also that distance apart. Likewise 
it is evident that with a four-cylinder crank 
shaft, pistons 1 and 4 travel together and also 
2 and 3 are coming up or going down together. 
The two usual ways for a six-cylinder engine to 
fire are 1, 5, 3, 6, 2, 4, and 1, 4, 2, 6, 3, 5. Here 
the throws are 120 degrees apart, and the pis- 
tons that travel together are 1 and 6, 2 and 5, 
and 3 and 4. V engines use the basic four- 
cylinder and six-cylinder firing orders to fire 
the two banks. The explosions will alternate 
between the two banks starting with the cylin- 
der at the pilot's end on the left block and fol- 
lowed by the forward cylinder on the right 
block. Explosions will occur on the left bank 
according to either one of the two firing orders, 
and those on the right bank in like manner 
except that on the right bank we will be work- 
ing from the propeller end toward the pilot's 
end. Where an engine is numbered LI, L2, 

[341 



ENGINE PARTS 

etc., and Rl, R2, etc., its firing order may be: 
LI, R6, L5, R2, L3, R4, L6, Rl, L2, R5, 
L4, R3. 

Where the left bank is numbered 1, 2, 3, 
etc., and the right bank 1', 2' ', etc., in the oppo- 
site direction, the firing order may be: 

1,1', 5,5', 3,3', 6,6', 2,2', 4,4'. 

The Curtiss OX with its peculiar cylinder 
numbering already referred to has the follow- 
ing distinctive firing order for normal rota- 
tion: 

1,2,3,4, 7,8,5,6. 

For an anti-normal engine it would be: 

2, 1,4,3,8, 7,6,5. 
or to start the cycle with an explosion in cylin- 
der No. 1 it would be: 

1,4,3,8, 7,6,5, 2. 

In order that the thrust exerted by the 
propeller upon the crank shaft may be trans- 
mitted to the crank case and then to the 
fuselage, a thrust bearing is placed upon the 
crank shaft very near the propeller hub. 
Thrust bearings are generally ball bearings hav- 
ing either one or two rows of balls and very often 
they are designed to take a load directed at 
right angles towards the center of the shaft as 
well as taking care of the thrust. In an engine 

[35] 



ELEMENTS OF AVIATION ENGINES 

like the Curtiss OX, where the crank shaft ex- 
tends several inches between the last main 
crank-shaft bearing and the propeller hub, the 
thrust bearing will be the last point where the 
shaft may be supported. Now if a shaft is 
allowed to revolve without a radial bearing at 
its end vibration will result and this must be 
avoided. Consequently on the Curtiss OX and 
all other engines having a nose, the thrust 
bearing must be capable of taking both radial 
and thrust loads. Some thrust bearings having 
a single row of balls will only take thrust in one 
direction. This makes it necessary to reverse 
the bearings if an engine is transferred from a 
tractor plane to a pusher plane or vice versa. 

The cam shaft is that part of the engine hav- 
ing irregularities upon its surface that open 
and close the valves at the proper time. The 
irregularities are called cams and are usually 
accurately shaped projections upon the shaft 
for imparting the necessary motion to a valve. 
Cam shafts are always made of high-grade 
steel and the cams are forged integral with the 
shaft. When gasoline engines were first being 
developed it was the practice to have as a cam 
shaft a plain piece of shafting with the cam 
keyed or pinned to it in the right position. 

[361 



on 




j 



///}LL-<SC0TT. 




CuRTISS OX 




Sturtevaut. 



THRUST BEARINGS 



ENGINE PARTS 

This resulted in an endless amount of cam- 
shaft trouble as the cam would often come 
loose causing a valve to operate at the wrong 
time or possibly not operate at all. Now that 
the cams and shaft are made in one piece, this 
difficulty is no longer encountered. 

The location of the cam shaft has been a 
matter of much discussion. The old practice 
was to have it located at the base of the cylin- 
ders as this was the most convenient position 
where T and L head cylinders were used. 
Since I head cylinders are more favorably 
looked upon, the overhead position of the cam 
shaft is being used more and more, as it does 
away with the numerous push rods used to 
operate the overhead valves. However, a cam 
shaft so placed necessitates a pillar shaft and 
bevel gears to drive it. V engines that use the 
base position of the cam shaft usually have the 
cylinders placed in a staggered position. This 
makes it much easier for one cam shaft located 
at the bottom of the V to operate the valves 
on both banks of cylinders. When a V engine 
uses the overhead position two cam shafts are 
necessary. 

In all four-stroke cycle engines the cam shaft 
always travels at half the crank-shaft speed. 

[37] 



ELEMENTS OF AVIATION ENGINES 

The reason is that it takes two revolutions of 
the crank shaft to complete a cycle and that 
the individual valves must open but once dur- 
ing a cycle. For instance, one cylinder will fire 
once during two revolutions of the crank shaft. 
In order that it may function, an inlet valve 
must open once to let a new charge in. Then 
the intake valve will open once during two 
revolutions of the crank shaft which means 
that the cam operating that valve must re- 
volve once to two revolutions of the crank 
shaft. 

Upon the valves depend to a great extent 
the success of the engine, for aviation engines 
seem particularly susceptible to valve trouble. 
The two general types of valves for gasoline 
motors are the poppet or mushroom type and 
the sliding sleeve type. The former is univer- 
sally used for aviation work largely because the 
latter type brings into account a little more 
weight. A poppet valve consists primarily of 
a disk with a bevelled edge and a stem joining 
the disk at its center. The bevelled edge is 
usually at an angle of 45 degrees with the plane 
of the disk, although other angles are not un- 
common. By having the valves open inwardly 
the force of an explosion or the force of a com- 

[38] 



ENGINE PARTS 

pression stroke will tend to push the valve 
firmly against the bevelled portion of the cy- 
linder referred to as the seat, and in this way 
the greater the force within the cylinder the 
more tightly will the valve be held in its closed 
position. It is safe to say that valves in avia- 
tion motors should be as large as possible. The 
use of I head cylinders restricts the size of the 
valves, so it is often impossible to put in a 
valve of a satisfactory diameter. T and L 
head cylinders permit the use of larger valves 
on account of the extension to the combustion 
chamber. The object of using large valves is 
simply to charge and scavage a cylinder more 
rapidly. 

When we consider under what conditions 
the valves must do their work, it will be seen 
why a great deal of attention has been paid to 
the materials of which they are made. The 
exhaust valve opens on the power stroke 
allowing the highly-heated gases to escape 
around it. Particles of carbon will invariably 
be carried outward and some will at times be 
caught between the valve and its seat at the 
instant it closes. The valve having been highly 
heated on account of its direct contact with 
the explosion, will be somewhat soft, and when 

[39] 



ELEMENTS OF AVIATION ENGINES 

it snaps against the particle of carbon a small 
indentation will be caused. This is called pit- 
ting, and to lessen it to a great extent it has 
now become the custom to make the exhaust 
valve of tungsten steel. Of the two valves the 
inlet is less subject to pitting, since the incom- 
ing gas tends to cool it, and furthermore less 
carbon collects on its seat. Nickel steel is the 
material sometimes used for inlet valves. 

In order to make a valve seat more firm after 
an engine has run considerably, and to prevent 
leaking, it is necessary to grind a new surface 
both on the valve and its seat in the cylinder. 
The abrasive is called grinding compound. It 
is applied as a very thin paste to either the 
valve or its seat, whereupon the valve is in- 
serted in its usual position and vigorously 
turned back and forth. If care is taken to fre- 
quently unseat the valve the compound will 
be kept evenly distributed over the grinding 
surface, and there will be little danger of cut- 
ting rings in either the valve or its seat. Prus- 
sian blue can be used to determine the fit. 
Frequently a valve may become warped or a 
shoulder may develop on the seat. A reamer 
can then be used to good advantage, but it 
must be followed by grinding. Sometimes the 

[40] 



ENGINE PARTS 

guide in which the valve stem works will be- 
come worn, making it useless to grind a valve 
until a bushing or new guide has been supplied. 

The springs used to close the valves deserve 
attention. On some engines double-coil springs 
are used, and then in case a spring breaks there 
will still be one to close the valve. Occasion- 
ally the exhaust valve springs will be a little 
heavier than those on the inlet valves. This is 
to allow for any decrease in strength caused by 
the heat from the exhaust valve and also to 
prevent any possibility of the exhaust valve 
being pulled down on the intake stroke. 

The ways in which the force of a revolving 
cam is brought to bear upon a valve stem are 
numerous and interesting. With an L head 
cylinder where the cam shaft runs directly 
under the valve, it is a simple matter to have a 
follower riding the cam and a tappet rod be- 
tween the follower and the valve stem. When 
the cam comes up the valve will be pushed up. 
As the cam goes on the spring will bring the 
valve back to its seat. This is simplicity itself, 
but the use of I head cylinders makes neces- 
sary other means of transmitting the cam 
thrust. 

The usual way of operating valves in the 

[41] 



ELEMENTS OF AVIATION ENGINES 

cylinder head by a cam shaft located at the 
base of the cylinders is to use push rods con- 
nected to rocker arms working on fulcrums 
attached to the tops of the cylinders. As the 
push rod is forced up, one end of the rocker 
arm goes up and the other end goes down, 
pushing inwardly on the end of the valve stem. 
This is the way the valves are operated on the 
Curtiss VX and the Sturtevant. The peculiar- 
ity in the Sturtevant is that the side thrust 
imparted to tappet is avoided by having a 
pivoted arm ride the cam and on this arm rests 
the tappet. Worn guides are reduced to a 
minimum in this way. 

The inlet valve operation on the Curtiss OX 
is interesting inasmuch as it brings into ac- 
count a new form of cam, and also because the 
valve is pulled open instead of receiving a 
direct thrust. Upon the cam shaft for each 
inlet valve are two cams that would be com- 
pletely circular but for a flat space on each. 
The space between these two cams is taken 
up by the exhaust valve cam, which is the 
ordinary type of cam. Upon the two round 
cams rests a tappet to which is attached a rod, 
or strictly speaking a tube, having a coil spring 
held about it at the top by a strap and at the 

[42] 




DIAGRAM TO ILLUSTRATE THE CURTISS OX VALVE ACTION 



ENGINE PARTS 
bottom by a collar upon the tube. The upper 
end of the tube is hinged to a lever arm that 
extends almost horizontally from a fulcrum 
upon the head of the cylinder, and directly 
under this lever arm is the end of the inlet 
valve stem. As the cam shaft revolves the flat 
spaces on the two cams will allow the spring to 
force the tappet and tube toward the center 
of the cam shaft, which results in a downward 
motion of one end of the lever arm. Since the 
inlet valve is located beneath it and since the 
spring on the tube is several times stronger 
than the spring upon the valve stem, the inlet 
valve is thus pulled open and remains open as 
long as the flat spaces will permit the spring to 
keep the tube in its downward position. It 
will be noticed that in this manner of opening 
the inlet valve everything depends upon the 
spring on the pull tube. 

Making the valves open and close at the 
right time on an engine or timing the valves, as 
it is generally called, is a simple matter pro- 
viding it is done systematically. The first 
thing to do is to select a cylinder, preferably 
No. 1 and, making sure a cam is not in a posi- 
tion to deliver a thrust, adjust the clearance 
of a valve on that cylinder by means of a feeler 

[43] 



ELEMENTS OF AVIATION ENGINES 

gage so that the clearance is that given by the 
manufacturer. Valve clearance varies between 
.010 and .030 inch, and its purpose is to allow 
for expansion of the stem and also to obtain a 
very accurate adjustment of a particular valve. 
It is the custom to time on the opening of an 
intake valve or on the closing of an exhaust 
valve. After the clearance has been adjusted 
for one valve and after the cam shaft gears 
have been unmeshed, the engine is turned in 
the direction it is intended to rotate until the 
piston in the selected cylinder is exactly in the 
right position for the inlet valve to open or for 
the exhaust valve to close. Then the cam shaft 
is revolved by hand in the direction it is in- 
tended to turn until the inlet valve is just 
starting to open or the exhaust valve has just 
closed as the case may be. The next step is to 
mesh the cam shaft gears. Sometimes the 
teeth come directly together and when that is 
the case it is necessary to "split a tooth." 
Different engines have ways of doing this, but 
it generally amounts to providing some means 
of revolving the gear wheel upon the cam 
shaft the distance of half a tooth, which is 
enough to allow the teeth to be meshed with- 
out disturbing the cam shaft. 

[44] 



ENGINE PARTS 

All of the other valves on the engine are 
timed by adjusting the clearance for each one. 
A piston is placed in the right position for a 
valve to open or close and if it does not func- 
tion correctly the clearance is changed until it 
opens or closes on time. From this it can be 
seen that if the cam shaft is out of time all 
valves will be affected, while if the clearance is 
set wrong it will affect only the valve having 
the wrong clearance. In other words, the cam 
shaft affects every valve, while clearance 
affects the individual valve. In cases where 
too much clearance is given above a valve stem 
the valve will open late and close early. When 
there is too little clearance, the valve will open 
early and close late. 

When spark plugs are placed in the cylinder 
head it is possible to determine the position of 
a piston at any time by removing a plug and 
inserting a steel scale. If valves are timed by 
determining a piston's position in this manner, 
it is spoken of as the linear method of timing. 
Inaccuracies may result from the use of this 
method where the pistons have convex heads 
and where particles of carbon are deposited on 
the piston heads. A more accurate method is 
to make use of a timing disk attached to the 

[45] 



ELEMENTS OF AVIATION ENGINES 

crank shaft near the propeller hub. The cir- 
cumference of this disk is divided into degrees 
with the points for opening and closing of each 
valve plainly marked upon it. If the disk is 
placed accurately upon the crank shaft it fur- 
nishes an excellent means of timing the valves, 
because no linear measurements need be taken. 
Since the angle of a crank throw must be used 
when working with a timing disk, this is called 
the angular method of timing valves. 



[46] 



CHAPTER V 
CARBURETION 

In order that gasoline may be mixed with 
the right amount of air to form an explosive 
mixture within the cylinders, it is necessary to 
make use of a device known as a carburetor. 
A great deal of attention has been devoted to 
the designing of carburetors, for it can be 
readily seen that the fuel consumption of an 
engine will be governed largely by the perform- 
ance of the carburetor. Also of late much 
attention has been given to the carburetion of 
lower grade fuels, so the subject of carburetors 
is becoming a broad field. 

A suitable mixture for an aviation engine is 
one pound of gasoline to fifteen pounds of air. 
A richer mixture would be one having more 
gasoline, while one having more air would be 
a leaner mixture. It has been found that the 
most practical way to obtain this mixture is to 
spray the gasoline into the air, and this is best 
accomplished by making use of a jet attached 
to a reservoir and lessening the atmospheric 
pressure about the jet. If the level of gasoline 
in the reservoir is slightly lower than the tip 

[47] 



ELEMENTS OF AVIATION ENGINES 
of the jet and the jet is located in an air-sup- 
plied chamber having a connection with the 
inlet valves, the downward motion of the pis- 
tons will result in less pressure being exerted 
upon the gasoline in the jet than that in the 
reservoir, where atmospheric pressure is ex- 
erted. Gasoline in this way will be made 
to flow from the jet, and since considerable 
air is being drawn past the jet it will tend 
to form a spray of the gasoline that is being 
delivered. 

To restrict the amount of gasoline that is 
supplied to the float chamber which in turn 
has a great deal to do with the gasoline deliv- 
ered by the jet, the float with which the float 
chamber is supplied, actuates a pin that opens 
and closes the main supply valve. Upon the 
top of the float rest the ends of two pivoted 
arms having the other ends in contact with 
the needle valve stem. As gasoline enters the 
float chamber the float will rise causing one end 
of the arms to rise and the other end to exert 
a downward pressure upon the needle valve. 
The result will be to seat needle valve allowing 
no more gasoline to enter until some has been 
drawn off by the delivery from the jet. From 
this it can readily be seen that the float cham- 

[48] 



CARBURETION 

ber is essentially a reservoir supplied with an 
automatic valve. 

The space around the jet is called the mixing 
chamber. To admit the necessary air an open- 
ing is located somewhere below the level of the 
jet which insures all of the air passing the jet. 
As a means of diverting the air nearer the tip 
of the jet and thus securing more of a drawing 
effect, the space around the jet through which 
the air passes is lessened by the insertion of a 
choke tube or a venturi as it is often called. 
Its purpose is to increase the velocity of air 
as it passes by the jet and thus increase the 
suction at the tip of the jet. To regulate the 
speed of the engine a butterfly valve is located 
just a little distance above the choke tube. 
This valve, which is nothing more than a disk 
of metal, is often referred to as the throttle. 
When it is opened the speed of the engine is 
increased on account of a greater volume of 
gas being taken by the engine. As it is brought 
toward a closed position, less gas will be sup- 
plied, and the result is to decrease the speed of 
the engine. Stop screws are provided to pre- 
vent the throttle from closing completely, for 
that would cause the engine to cease running 
altogether. 

[49] 



ELEMENTS OF AVIATION ENGINES 

So far the most elementary type of carbu- 
retor has been discussed. It is one that consists 
primarily of a float chamber and one jet in a 
regularly shaped mixing chamber. This is 
called a simple jet carburetor, and its chief 
weakness lies in the fact that at high speeds it 
will deliver a richer mixture than when the 
engine is running slowly; the reason for this 
being that as the speed is increased the suction 
is greatly increased, which means more gaso- 
line in proportion to the air at high speeds than 
at low speeds. A simple jet carburetor ad- 
justed for low speeds will use too much gaso- 
line at high speeds, while one that is adjusted 
for high speeds will not supply enough gasoline 
at low speeds. Consequently simple jet car- 
buretors are not satisfactory for aviation en- 
gines. 

In order to secure the right mixture at both 
low and high speeds, several modifications of 
the simple jet carburetor have been used with 
more or less success. One way is to have the 
mixing chamber supplied with an auxiliary air 
valve that is held in place by a weak spring. 
At low speeds the spring holds the valve closed, 
but as the speed is increased the valve is 
drawn open due to the increase in suction. 

[50] 



r? 




THE MILLER AVIATION CARBURETOR 



CARBURETION 

This allows more air to enter the mixing cham- 
ber at high speeds causing the mixture to be- 
come slightly leaner and thereby securing 
approximately the same mixture at high speeds 
as at low speeds. Another way is to have the 
opening in the jet supplied with a metering pin 
which is nothing more than a slender pin 
tapered to a point that extends within the jet. 
As the throttle is opened, the metering pin is 
withdrawn much more slowly proportionately 
than the throttle is turned. This will allow a 
slightly greater amount of gasoline to issue 
from the jet at high speeds than at low speeds, 
but if arranged correctly the increase in gaso- 
line will be in proportion to the increase in air. 
A third way is to employ several jets instead 
of one, and by using a rotary throttle uncover 
them one at a time as the speed is increased, 
allowing the air to pass by more than one. In 
this manner at high speeds more jets are ex- 
posed to the suction than at low speeds, and 
likewise the size of the air opening is larger at 
high speeds than at low speeds. A uniform 
mixture for all speeds is thus secured. A 
fourth way is to combine two small jets so that 
one will deliver more and more gasoline as the 
speed is increased and the other will deliver 

[51] 



ELEMENTS OF AVIATION ENGINES 

only a limited amount. At high speeds the in- 
creased amount of gasoline from one will be 
just enough to take care of the additional 
amount needed. At low speeds both jets work 
harmoniously. Such departures from the sim- 
ple jet carburetor are spoken of as speed com- 
pensations. 

The Zenith carburetor has been widely used 
in connection with aviation engines, and for 
that reason it will be well to become familiar 
with its operation. The principle used is that 
of two small jets with one having only a lim- 
ited amount of gasoline to supply. In appear- 
ance it closely resembles a simple jet carbu- 
retor except for a narrow cylindrically-shaped 
well between the float chamber and the mixing 
chamber. Gasoline is supplied from the float 
chamber to this well through a small hole in a 
plug that forms the bottom of the well. The 
plug is called the compensator. In the upper 
part of the well is a hole which allows atmos- 
pheric pressure to be exerted upon the gasoline 
within. One jet is placed within the other, and 
the inside jet is that one connected directly to 
the float chamber. Obviously this jet, which is 
known as the main jet, will act the same as one 
in a simple jet carburetor causing a richer mix- 

[52] 




jUjllif{^»M 



A HALF SECTION VIEW OF A ZENITH CARBURETOR 



CARBURETION 

ture at high speeds than at low speeds. The 
outside jet or cap jet, as it is called, receives its 
supply of gasoline from the well, and since the 
amount of gasoline furnished to the well is 
limited by the hole in the compensator, it can 
be seen that the amount of gasoline delivered 
by the cap jet is restricted to that amount that 
will flow by gravity through the hole in the 
compensator. At low speeds both jets work 
normally, but as the speed is increased the 
main jet will furnish more and more gasoline 
while there will be a tendency to draw more 
gasoline from the cap jet than can be supplied 
by the hole in the compensator. The result 
will be to exhaust the supply in the well and 
use instantly that which is fed to it. Since 
there is an air hole near the top of the well un- 
due suction upon the compensator will be pre- 
vented. It should be noted that air will enter 
the well and be drawn out the cap jet at very 
high speeds, but it is absolutely wrong to re- 
gard the air hole in the upper part of well as an 
auxiliary air valve. The compensation effect 
comes from the fact that the increased amount 
of gasoline supplied by the main jet is enough 
to make up for that which is not supplied by 
the cap jet. 

[53] 



ELEMENTS OF AVIATION ENGINES 

At idling speed very little air is drawn in, 
and this is not sufficient to fully overcome the 
surface tension of the gasoline in the jets. By 
surface tension is meant the force that tends 
to resist breaking the surface of the column of 
gasoline in the jet and drawing it outward. To 
insure a good mixture at idling speed the 
Zenith is equipped with an entirely separate 
carburetor that supplies its gas at a point 
where the air passes by the nearly closed 
throttle, and, on account of the small space, 
considerable suction is developed at this point. 
This carburetor gets its supply of gasoline 
through a tube leading down near the bottom 
of the well. The tube is held in what is called 
the priming plug, which acts as a cover for the 
well. The size of the hole in the priming plug 
governs the amount of gasoline fed to the 
idling carburetor. The amount of air that is 
allowed to enter the mixing chamber of the 
idling carburetor is controlled by a thumb 
screw known as the slow-speed screw. 

To facilitate starting, a strangler valve is 
placed in the air inlet. If it is brought toward a 
closed position a greatly increased amount of 
gasoline will be drawn from the jets, and from 
this increased amount the more readily vola- 

[541 



CARBURETION 

tile parts will go to furnish a combustible mix- 
ture. The strangler is used only in starting. 

The variables are those parts affecting the 
mixture which can be replaced by similar ones 
having different dimensions. In naming them 
they are generally given in a regular order be- 
ginning with the choke tube, then the main jet, 
the compensator, and finally the priming plug. 
The cap jet is not a variable. Zenith settings 
comprise the internal diameter of the variables. 
The choke tube is measured in millimeters and 
the other three in hundredths of a millimeter. 
The size of a carburetor is the diameter in 
inches of its connection with the manifold. 

The adjustments on the Zenith are the 
throttle stop screw, which governs the suction 
upon the idling carburetor, the slow-speed 
screw, to adjust the priming plug's delivery, 
and adjusting the level of gasoline in the float 
chamber by changing the position of the 
needle valve seat. This is accomplished by 
adding washers under the seat if the level is to 
be lowered or by withdrawing washers if the 
level is to be raised. 

As a plane goes from a low altitude to a 
higher one the effect upon the carburetor will 
be to furnish a richer mixture, since the same 

[55] 



ELEMENTS OF AVIATION ENGINES 

volume of air will be used, yet its weight will 
be appreciably decreased. One way of com- 
pensating for altitude is to have an air valve 
located in the manifold that can be opened by 
the pilot as necessity calls for. The effect of 
opening this valve will be to allow a little air to 
be added to the rich mixture. Another method 
is to decrease the pressure upon the level of 
gasoline in the float chamber by opening a 
valve in a tube leading from the float chamber 
cover to the manifold. The reduced pressure 
within the manifold in this way is used to 
slightly reduce the atmospheric pressure upon 
the gasoline in the float chamber. The effect 
will be to cause less difference in pressure be- 
tween the gasoline in the float chamber and 
the gasoline in the jets, resulting in a decrease 
in the amount of gasoline delivered at the jets. 



[56] 



CHAPTER VI 
IGNITION 

The electric spark, which is the only sat- 
isfactory means of igniting a charge of gas 
in an internal combustion engine, is furnished 
by current coming from batteries or a mag- 
neto. In a battery electricity is generated by 
chemical action while the magneto is a me- 
chanical means of generating electricity. Al- 
though the care of a battery is important, there 
is no call for an extensive knowledge of bat- 
tery construction to keep it in good condition. 
With a magneto, however, there are many 
moving parts which need attention and fre- 
quently adjustments are necessary, so it seems 
advisable to take up the magneto rather fully. 
To start with the fundamentals of electricity 
it will be remembered that if a coil of wire is 
revolved between the poles of a horseshoe mag- 
net so that it cuts the lines of magnetic force 
there will be a current generated in the wire 
that goes to make up the coil. Furthermore, if 
this coil is wound about a piece of soft iron 
known as a core, and the core revolved be- 
tween the two magnetic poles so that magnet- 

[57] 



ELEMENTS OF AVIATION ENGINES 

ism passes through the core one instant and 
not the next, then more electricity will be gen- 
erated. The core offers an easy path for the 
magnetism. Soft iron is used for the core be- 
cause it can very quickly become magnetized, 
and what is just as important it will quickly 
give up its magnetism. By revolving such a 
core between the poles of a horseshoe magnet, 
it will amount to successively plunging a mag- 
net in a coil and rapidly drawing it out again. 
The magnetic lines of force from the core, 
when it is magnetized, will of course be cut by 
the coil which accounts for the current. 

As the core is revolved between the two 
magnetic poles, which are distinguished by 
calling one the North pole and the other the 
South pole, the core is magnetized when in a 
horizontal position almost connecting the two 
poles and demagnetized when it has turned 
90 degrees to a vertical position. Consequently 
in one complete revolution of the core it will 
be magnetized twice and demagnetized twice. 
It so happens that a little more current is gen- 
erated in the coil when the core loses its mag- 
netism than when it receives its magnetism, 
which means that maximum current is ob- 
tained when the core is approximately in a 

[58] 




7*osmoN or Core When 
Monetized 




POSITIOU OF CORE WHEN 

Primary Circuit Is Proken. 



DIAGRAMS TO ILLUSTRATE THE LOCATION OF THE CORE IN A 
SHUTTLE TYPE MAGNETO 



IGNITION 

vertical position. Since it is in this position 
twice during one complete revolution, it fol- 
lows that an ordinary magneto furnishes two 
sparks per revolution. 

So far the core has been considered as the 
revolving part. Identically the same result is 
obtained when the core is held stationary and 
the magnet or magnets are revolved. To turn 
the magnets is inconvenient on account of their 
horseshoe shape, so rotating poles are often 
used to accomplish the same result. This is 
referred to as a revolving field. In the first 
case where the core is rotated, an armature 
made up of the core and the shaft that carries 
it is used. In appearance the armature has 
somewhat of a resemblance to a shuttle, on 
account of the windings about the core. For 
this reason the type of magneto using the re- 
volving core and coil is called the shuttle type, 
while the one in which the magnetic field re- 
volves and the coil remains stationary is known 
as the inductor type. More attention will be 
devoted to the inductor type after the shuttle 
type has been further explained. 

The current required to jump the gap be- 
tween the points of a spark plug under high 
compression is much greater than the amount 

[59] 



ELEMENTS OF AVIATION ENGINES 

supplied by a single coil wound about a core. 
In order to have a self-contained unit it is 
necessary to make use of another coil wound 
about the first consisting of much finer wire 
and having several hundred times as many 
turns as in the first coil. The coil wound near- 
est the core is called the primary coil, while the 
outside one is the secondary coil. Now if we 
have some automatic device to break the path 
of the current from the primary coil at the 
same time that the core loses its magnetic 
charge a high tension current will be induced 
into the secondary coil and will be suitable to 
conduct to the spark plugs. The principle is 
that of a transformer. 

On the shuttle type magneto a breaker me- 
chanism through which the primary current 
passes is held on one end of the armature, 
causing it to be revolved at exactly the same 
speed as the armature is turning. Cams on the 
breaker housing force the breaker points to 
separate for an instant, at the same time that 
the core loses its magnetism. All that is neces- 
sary to do in order to stop the magneto from 
delivering current and in turn stop the engine 
is to close a switch on a line that connects the 
two breaker points. This will short circuit 

[60] 



IGNITION 

the primary, destroying the effectiveness of the 
breaker points and prevent the primary from 
inducing any current into the secondary coil. 

To advance or retard the spark, the position 
of the breaker cams is changed. This affects 
the time that the primary circuit is broken. 
Moving the cams with the direction of rota- 
tion retards the spark, while to advance the 
spark the cams are moved against the direc- 
tion of rotation. This brings us to one diffi- 
culty with the shuttle type magneto. In order 
to get the maximum current the primary circuit 
should be broken as the core loses its magnetic 
charge. If the spark is retarded, however, the 
primary is broken a little later than the mag- 
netic lines of force are broken, which results in a 
weaker spark. The effect is frequently to hind- 
er starting as it is necessary to retard the spark 
to prevent injuring the one who is cranking. 

In the primary circuit a condenser is placed 
in multiple with the breaker points. It con- 
sists of alternate sheets of a conductor and a 
non-conductor such as tinfoil and mica. Half 
of the sheets of the conductor are attached to 
one terminal, and the other half are attached 
to the second terminal. This provides a place 
for the current to go momentarily after the 

[61] 



ELEMENTS OF AVIATION ENGINES 

breaker points have separated. It a condenser 
were not used there would be a tendency for 
the current to continue flowing for an instant 
through the air between the separating points, 
which would result in arcing and pitting the 
points. Right here it should be noted that the 
breaker points are of platinum and should not 
separate more than .020 inch. Since the con- 
denser prevents arcing it also serves to make 
the break in the primary circuit occur more 
quickly, which means that more voltage will 
be induced into the secondary coil. 

As a means of conducting the secondary cur- 
rent to the right spark plug at the right time a 
distributor is used. It consists of as many seg- 
ments as the number of spark plugs that the 
magneto supplies. A distributor arm with a 
carbon brush directly connected with the sec- 
ondary coil turns about upon the distributor 
plate conducting secondary current to each 
segment in turn. With the spark fully ad- 
vanced, the distributor arm should just be 
entering a segment every time the breaker 
points separate. For convenience the primary 
and secondary circuits are both grounded. 
Should the secondary circuit be left open, as 
would be the case if a wire were not attached 

162 1 




WIRING DIAGRAM OF A MAGNETO SYSTEM 



IGNITION 

to a spark plug, the result might be that the 
high pressure of the secondary would cause a 
short circuit between the two coils. To avoid 
such a happening a safety gap is provided in 
the secondary circuit. Its points are generally 
three-eighths of an inch apart insuring no in- 
terference with sparking at the plugs. 

The electrical pressure is expressed in volts. 
The flow is expressed in amperes. One volt 
times one ampere is equivalent to one watt, 
which is nothing more than a unit of work, 
being 1 /764 part of a horse-power. The wattage 
of an ordinary magneto is about twenty. The 
voltage in the primary circuit is from six to ten 
volts, while that in the secondary is about ten 
thousand. The amperage of the primary is 
limited to only a few amperes, yet that of the 
secondary is infinitely less, being only the 
slightest fraction of an ampere, for it should be 
remembered that when a current of higher 
voltage is obtained by induction the gain in 
the number of volts will be accompanied by a 
loss in the number of amperes. Upon the speed 
of the armature and the number of windings 
depends the voltage of a magneto. The num- 
ber of amperes is dependent upon the strength 
of the magnets. 

[63] 



ELEMENTS OF AVIATION ENGINES 

An ordinary magneto can deliver but two 
sparks per revolution, so the speed of the arma- 
ture is governed by the number of explosions 
in the engine during one complete cycle. A 
four-cylinder engine will fire four times in two 
revolutions or two times in one revolution. 
Since two sparks will then be necessary for 
every revolution of the crank shaft, it follows 
that the armature should turn at engine speed. 
In an eight-cylinder engine there will be four 
explosions per revolution, so the armature will 
have to turn at twice engine speed to give the 
four sparks at the right time. The speed of an 
armature on a magneto supplying twelve 
cylinders would be three times engine speed. 
A convenient means of determining this rela- 
tive speed is to divide the number of cylinders 
by four. The distributor arm turns at cam- 
shaft speed owing to the fact that each cylin- 
der requires one spark in two revolutions of the 
engine. 

The Dixie magneto, which is a good example 
of the inductor type, has been widely used and 
deserves consideration. In it the magnets are 
turned at right angles to the position that they 
occupy in the Bosch and Berling, which are 
representatives of the shuttle type. A shaft 

[64] 




N 



N 



BRONZE 



DIAGRAM TO ILLUSTRATE THE PRINCIPLE OF REVOLVING 
POLES ON THE DIXIE MAGNETO 



IGNITION 

carrying two shoes or pole extensions separated 
by a bronze block is placed in line with the two 
poles of the magnets. This shaft having no 
windings upon it is not called an armature, 
but is known as a rotor. As the rotor is re- 
volved the shoes, each being in contact with 
one pole and being separated by the non-mag- 
netic bronze, will always have their respective 
magnetic charges, and the effect will be much 
the same as though the magnets themselves 
were revolved. Were it not for the bronze be- 
tween the two shoes there would be a direct 
flow of magnetism through the rotor between 
the two poles, and the shoes would then be 
useless. 

At right angles to the rotor is placed the core 
carrying the primary and secondary coils. It 
is located in the space between the rotor and 
the top of the magnets. Extending downward 
from both ends of the core are two bars of soft 
iron known as field pieces, and it is between 
these two field pieces that the shoes revolve. 
When the shoes are in a horizontal position, 
magnetism will pass from one shoe into the 
nearest field piece, then through the core, into 
the other field piece, and thence into the oppo- 
site shoe. When the shoes move to a vertical 

[65] 



ELEMENTS OF AVIATION ENGINES 

position the core will receive no magnetism, 
but in moving another 90 degrees the shoes 
will come again to a horizontal position, and 
magnetism will pass through the core in a re- 
versed direction. Thus the core will be mag- 
netically charged one instant and not the next, 
resulting in the generating of electricity. 

The breaker assembly does not revolve on 
the end of the rotor, but is worked by cams on 
the end of the rotor shaft. To advance or re- 
tard the spark it is thus possible to move the 
whole breaker assembly instead of changing 
the position of fixed cams, as is done on the 
shuttle type. Since the coils and core do not 
revolve, it is also possible to change the posi- 
tion of the core and field pieces with the 
changing of the position of the breakers. The 
result is to break the magnetism in the core 
with the breaking of the primary circuit even 
though the spark is fully retarded. This in- 
sures the same intensity of spark when crank- 
ing the engine as is obtained at top speed with 
the spark fully advanced. 

A special type of Dixie magneto is one hav- 
ing four shoes instead of two. There are two 
opposite North shoes and two opposite South 
shoes. The two field pieces leading to the core 

[661 




DIAGRAM TO ILLUSTRATE POSITION OF ROTOR IN THE DIXIE 
MAGNETO WHEN THE CORE IS MAGNETIZED 




DIAGRAM TO ILLUSTRATE POSITION OF ROTOR IN THE DIXIE 
MAGNETO WHEN THE CORE IS DEMAGNETIZED 



IGNITION 

are shortened so that their ends will be well 
above the center of the rotor to allow unlike 
shoes to connect the two ends. Since the oppo- 
site shoes have the same polarity, it would not 
do to have the ends of the field pieces in line 
with opposite shoes. The advantage of this 
type of Dixie is that four sparks can be secured 
during one revolution of the rotor, which per- 
mits a much slower running magneto on an 
engine having a large number of cylinders. 

The magnets themselves deserve attention. 
They are made of hard steel in order to retain 
their magnetism as long as possible. To re- 
charge a magnet it is simply necessary to wind 
it with insulated wire and pass direct current 
through the wire. When dismounted from the 
magneto it is necessary to provide a path for 
the magnetism between the two poles. A strip 
of steel will answer for a keeper, or the unlike 
poles of two magnets may be placed together, 
insuring a perfect magnetic circuit. 

In timing a magneto to an engine it is best 
to start by selecting a cylinder and placing the 
piston in that cylinder in the right position on 
the compression stroke for the spark to occur. 
Then turn the distributor arm so that it is 
about to enter the segment which has connec- 

[67] 



ELEMENTS OF AVIATION ENGINES 

tion with the spark plug in the selected cylin- 
der. Next, after making sure the spark lever 
is fully advanced, turn the magneto in its right 
direction until the breaker points have just 
separated. The distributor arm should now be 
upon the foremost part of the segment to in- 
sure that it will still be in contact with the seg- 
ment when the spark lever is retarded. The 
remaining step is to connect the driving shaft 
with the armature or rotor as the case may be. 
Upon those engines having double ignition to 
procure a greater factor of safety and to reduce 
the time to fully explode the charge, the two 
magnetos must furnish their sparks at the same 
time or be synchronized as it is technically 
called. To accomplish this the first magneto is 
timed to the engine and the second magneto is 
timed to the first. In this way the breaker 
points on each one can be made to separate at 
the same instant. 

With a battery system either a vibrating or 
a non- vibrating coil may be used. Vibrating 
coils will give a rapid succession of sparks at 
the spark plugs. The primary circuit is made 
to pass through the vibrator and the magnet- 
ism in core is allowed to separate the vibrator 
points which breaks the primary circuit and 

[681 




DIAGRAM OF A BATTERY SYSTEM OF IGNITION 
WITH A NON VIBRATING COIL 



IGNITION 

demagnetizes the core. The contact is then 
made again by the vibrator springing back and 
the operation is repeated. When a non- vibrat- 
ing coil is used there must be mechanically- 
operated breakers to break the primary circuit, 
very similar to those used on magnetos. 

The wiring diagram for a battery system 
using mechanically-operated breakers is simi- 
lar to a magneto wiring diagram, except that 
the switch is not placed in the same position. 
In a battery system the switch is placed in 
series in the primary circuit, and by opening 
the switch the engine is stopped. Sometimes 
two breakers are used instead of a single one. 
The two are then wired in multiple and are 
made to break at the same time, thereby in- 
suring uninterrupted flight in case one refuses 
to close. A small resistance coil of iron wire 
is often placed in the primary circuit with a 
view to saving the battery during slow run- 
ning, or in case the switch is left closed when 
the engine is not running. Ordinarily when 
the engine stops the breaker points are to- 
gether, which, with a closed switch, affords a 
direct path for the current to pass from one 
pole of the battery to the other. The iron coil 
will then be heated with the result that less 

[691 



ELEMENTS OF AVIATION ENGINES 

current can pass through the heated iron wire 
than when it was cold. In this way a battery 
will not be exhausted so readily. A coil that 
serves this purpose is called a ballast coil. 



[70] 



CHAPTER VII 
LUBRICATION 

The purpose of lubrication is to reduce 
friction. Even though two pieces of metal 
that move one upon the other may have their 
surfaces highly polished and appear perfectly 
smooth, it will be noticed upon examination 
with a microscope that the surfaces are very 
irregular. In other words all sliding surfaces, 
no matter how carefully they may be finished, 
are known to consist of minute projections and 
depressions. Consequently the projections and 
hollows on the contact faces tend to interlock 
and resist a sliding motion. From this it can 
readily be seen that friction is nothing more 
than the force which resists the relative motion 
of one body in contact with another body. 
Excessive friction results in the development 
of heat. 

As a means of minimizing friction, oil is in- 
troduced between the contact surfaces. The 
oil will first fill the depressions and finally 
form a film between the two surfaces, separat- 
ing them sufficiently to prevent the projections 
on one surface from interlocking with the de- 

[71] 



ELEMENTS OF AVIATION ENGINES 

pressions on the other. This is referred to as 
the theory of lubrication. Perfect lubrication 
is greatly to be desired for it eliminates wear, 
and by reducing the power required to turn the 
engine it adds to the efficiency of an engine. 

After realizing the necessity for oil the next 
step is to ascertain what properties an oil must 
have in order that it may be suitable for avia- 
tion engines. In testing an oil it is customary to 
determine the gravity, viscosity, flash point, fire 
point, and whether or not it has acid properties. 

The gravity of an oil has in reality no effect 
upon its lubricating merits, as there is con- 
siderable variation in the gravity of high grade 
oils. However, it is usually determined and 
used principally in checking current deliveries 
of a certain brand. The specific gravity is the 
ratio between its weight and the weight of an 
equal volume of water. I n the oil trade, though, 
it is customary to use the Baume gravity scale 
in which the gravity of water is 10 at 60° F. 
The lighter the oil is in body, the higher will 
be the Baume reading. Hydrometers gradu- 
ated for either specific gravity or Baume are 
used to measure the gravity of an oil. The fol- 
lowing formula will serve to convert one scale 
in another: 

[721 



LUBRICATION 

Specific gravity =, 



130+Baume reading* 

Viscosity is the technical name for what is 
popularly called "body." To express it more 
specifically it is the fluidity of an oil. To ob- 
tain the viscosity the oil is put into a cup sur- 
rounded by water at about 212° F. When the 
oil has reached this temperature, a plug of 
specific size in the bottom of the cup is re- 
moved allowing 60 c.c. of the hot oil to runout 
into a marked flask. The number of seconds 
required to draw the 60 c.c. is reported as the 
viscosity of the oil. Good cylinder oil will have 
a viscosity of about 75 seconds. 

The flash point is the lowest temperature at 
which the oil will ignite but not continue to 
burn. If the flash point is too low, the oil will 
not remain on the cylinder walls and bearings 
when the normal heat is developed, leaving the 
friction surfaces without lubrication. It is well 
to use oil having a flash point above 325° F. 

The fire point is the temperature at which 
the ignited vapor from the oil will continue to 
burn. This temperature, which ranges be- 
tween 45° and 75° F above the flash point, is 
not of much consequence from our standpoint 
as it is always beyond the point where the oil 
will cease to be useful. 

[73] 



ELEMENTS OF AVIATION ENGINES 

Certain mineral oils are treated with sul- 
phuric acid during the process of refinement. 
To protect the highly polished bearing sur- 
faces it may be necessary to ascertain if any 
acid has remained in the oil. A simple way of 
testing is to wash a sample of the oil with 
warm water and test the water with litmus 
paper. The presence of any acid will result in 
the paper being turned pink. 

Lubricating oil we are accustomed to think 
of as being only mineral oil. With the develop- 
ment of aviation engines, castor oil, which is a 
vegetable oil, has received considerable atten- 
tion. This oil, which has a gravity of 96° 
Baume and a flash point a little higher than 
most mineral oils, will thicken to a marked de- 
gree upon standing. When heated it will 
readily oxidize and exhibit acid properties, 
rendering it of little use in engines where the 
oil is used over and over again. Its universal 
use in rotary engines is due to the fact that it 
will not unite with gasoline. In these engines 
the crank case is filled with gasoline vapor which 
tends to wash off any mineral oil that is supplied 
to the bearings. Hence castor oil is resorted 
to, and as long as the oil is used but once in 
rotary engines, it serves very well as a lubricant. 

[74] 



LUBRICATION 

Very few engines have the same system of 
lubrication. The oil supply is generally carried 
in the lower half of the crank case which is 
called the sump. Frequently auxiliary tanks 
having connection with the sump are used. 
The splash system of oiling is not suitable for 
aviation engines, so it is possible to make use 
of what is called a dry sump. If no provision 
is made to retain the oil in the sump when an 
engine is momentarily inverted there is great 
danger of the oil rushing into the cylinder and 
piston cavities. However, if the lower half of 
the crank case has a false bottom the oil may 
be carried in the compartment thus formed 
with no danger of it rushing out. Another way 
to obtain a dry sump is to collect the returning 
oil from the bearings in a trap at the bottom of 
the crank case and pump it away to a tank 
where the main supply is located. 

A gear pump is generally used to force the 
oil to the bearings. Its construction is remark- 
ably simple as it consists of two rotating gears 
in a closely-fitted housing. Oil is caught in the 
spaces between the successive teeth of each 
gear and carried around to the discharge of the 
pump. Plunger pumps and vane pumps are 
also used. The Hispano-Suiza engine makes 

[75] 



ELEMENTS OF AVIATION ENGINES 

use of the vane pump to develop a high oil 
pressure. 

A pressure relief valve is usually placed on 
the oil line very near the pump. Such a valve 
consists essentially of a poppet valve which 
opens outwardly and which is held in place by 
a spring fitted with a cap screw. In case the 
line were to become obstructed, the valve will 
open relieving the pressure and permitting the 
oil to return to the sump. By changing 
the tension of the spring with the cap screw the 
oil pressure may be regulated. A sight gage 
in the cock pit is used to indicate the pressure. 
It should be connected to the main oil line at 
a point not far distant from the pump. When 
plunger pumps are used, pulsators are often 
employed to show the operation of the pumps. 
A pulsator consists of a glass dome in which a 
quantity of air is compressed by the entrance 
of some oil from the main oil line. The im- 
pulses of the plunger in the pump will give rise 
to a throbbing motion of the surface of oil in 
the glass dome. 

While the oiling system in the Curtiss OX 
can not be regarded as representing the way 
all aviation engines are oiled, it may be well to 
describe it and point out its peculiarities. In 

[76] 




GEAR PUMP 




DIAGRAM TO ILLUSTRATE THE OPERATION OF A VANE PUMP 



LUBRICATION 

this engine the oil is carried in the sump where 
it is covered with splash pans. Toward the 
propeller end of the sump is located a gear 
pump which forces the oil under a pressure of 
about fifty pounds to the propeller end of the 
hollow cam shaft. At each of the five cam- 
shaft bearings a hole is drilled allowing oil to 
escape and oil these bearings. Directly be- 
neath each cam-shaft bearing is a crank-shaft 
bearing. The partitions in the crank case or 
webs as they are called, which connect the 
cam-shaft bearings with the crank-shaft bear- 
ings, are drilled. In this way oil is supplied to 
grooves in the cam-shaft bearings, whence it 
is forced down the holes in the webs to the 
crank-shaft bearings. Radially-drilled holes in 
the hollow crank shaft at each of the five main 
bearings allow oil to pass once in each revolu- 
tion from the holes in the crank case webs into 
the crank shaft. Centrifugal force carries it 
out the crank-shaft throws. Each crank pin 
has two holes drilled in that part of the pin 
directly away from the center of rotation. In 
this way centrifugal force carries oil out of the 
crank pins to oil the two lower connecting-rod 
bearings upon each crank pin. The seepage 
from these bearings develops a spray of oil 

[77] 



ELEMENTS OF AVIATION ENGINES 

within the crank case. As a piston moves up- 
ward, exposing part of the cylinder wall, the 
spray comes in contact with the wall and oils 
it. Also the spray is made use of to oil the 
wrist-pin bearings in the piston bosses by 
allowing it to enter holes drilled in the bosses. 
Since the cam shaft is located within the crank 
case in this engine the cam surfaces will be 
oiled by the spray. The magneto gear and cam 
shaft gear are oiled by spray from a hole in the 
retaining screw of the cam-shaft gear. The 
thrust bearing receives what little oil it re- 
quires from a small hole in the crank shaft near 
this bearing. The most peculiar feature of the 
system is that the cam shaft is used as a main 
distributing line. A crank shaft hollow through- 
out is another striking feature. 

Some aviation engines are equipped with 
cooling devices for the oil. Their object is to 
maintain a suitable viscosity and also a slight 
cooling effect upon the bearings. The Hall- 
Scott engine makes use of an oil-cooling jacket 
very near the carburetor on the intake mani- 
fold. Owing to the fact that vaporization is 
accompanied by the extraction of heat from 
surrounding bodies, the oil is cooled by the 
vaporizing of gasoline. The Thomas-Morse 

[78] 



LUBRICATION 

engine passes its oil through a coil of pipe 
known as an oil radiator which is located in a 
sufficiently cool place. An auxiliary oil tank is 
used in connection with the Sturtevant engine. 
By passing the oil to and from the tank a low- 
ering in temperature is secured. Another 
method used on some foreign engines is to pass 
air tubes through the sump, and, by drawing 
air through these tubes to the carburetor, a 
cooling effect upon the oil in the sump will be 
gained. 



[79] 



CHAPTER VIII 
COOLING 

A rapid succession of explosions within a 
cylinder would soon heat its interior to 
redness if some means were not taken to con- 
duct the heat away. Such high temperature 
would burn the lubricating oil and cause the 
pistons to seize and the bearings to burn out. 
Excessive heat will also cause irregularities in 
the combustion chamber to become so highly 
heated that they will ignite a fresh charge of 
gas. Obviously some of the heat must be con- 
ducted away. A great danger comes into 
account at this point, because it often happens 
that too much heat is removed. If the cylinder 
is too cool the pressure of the gas expanding 
during a power stroke will be lessened by a 
large amount of its heat entering the cylinder 
wall, for it will be remembered that the pressure 
exerted by a gas is governed largely by tem- 
perature. From this it can be seen that there 
is a great necessity for cooling, but to get the 
maximum efficiency from an engine it likewise 
is necessary to avoid over cooling. 

Heat may be conducted from a cylinder by 

[80] 



COOLING 
water or by air. Fixed-cylinder engines with one 
or two exceptions are always water cooled, 
while rotary engines are invariably air cooled. 
The advantages in air cooling are a decrease in 
weight and the avoidance of a circulating sys- 
tem that can be pierced by bullets. However, 
uniform cooling can best be accomplished by 
water. The most important point to note re- 
garding air-cooled engines is that the cylinders 
are supplied with cooling flanges which in- 
crease the surface from which the heat may be 
radiated. 

Water-cooled engines make use of a radia- 
tor, usually cellular though sometimes tubular 
in construction, and water jackets upon the 
cylinders. Water is supplied to the base of the 
jackets and moves upward over the heads of 
cylinders. At the top of each cylinder it is con- 
ducted to the top of the radiator where it is 
cooled and consequently tends to move to- 
ward the base of the radiator. From this point 
the relatively cool water is drawn to the pump 
which delivers it to the water jackets to be 
heated again. In this way the water is circu- 
lated by the pump in the same direction that 
the constant heating and cooling would cause 
it to travel. A thermosyphon system would be 

[81] 



ELEMENTS OF AVIATION ENGINES 

one depending wholly upon automatic circula- 
tion from this source. Oftentimes to prevent 
condensation within the intake manifold a 
little of the hot water that is about to enter the 
top of the radiator is led through a jacket on 
the manifold and from there to the intake of 
the pump. 

The type of water pump used on aviation 
engines is with few exceptions the centrifugal 
pump. In cases where these are not used the 
gear pump is employed. In a centrifugal pump 
the water is led into the center of a circular 
chamber in which revolve several blades on a 
common shaft. A whirling motion of the water 
is secured enabling it to be led away at a 
tangent to the circular chamber under pres- 
sure. One advantage of a centrifugal pump 
over a gear pump is that the water may circu- 
late through the pump after it has stopped 
operating. When a gear pump ceases to oper- 
ate the water system is blocked. 

Hose connections are used on the water lines 
between the engine and the radiator. These 
prevent many of the vibrations of the engine 
from reaching the radiator. In making hose 
connections, especially on the line leading to 
the intake of the pump, care should be taken 

[82] 




CENTRIFUGAL PUMP 



COOLING 

not to have more than one and one-half inches 
of hose exposed to the water, as there is danger 
of the hose weakening and lessening the flow 
by its being drawn together. 

When a plane is to be used in winter weather 
or when it is required to fly at a great altitude, 
anti-freezing mixtures may be used. The best 
one consists of 17 per cent alcohol, 17 per cent 
glycerine, and 66 per cent water. Such a mix- 
ture will be suitable at a temperature as low as 
15° below zero. Although this mixture is far 
superior to salt solutions it is not perfectly sat- 
isfactory, because after being heated there is 
always an uncertainty as to the quantity of 
alcohol. 



[83] 



CHAPTER IX 
ROTARY ENGINES 

A rotary engine is one in which the cylin- 
ders and crank case revolve about a sta- 
tionary crank shaft. The common type is that 
having the cylinders placed in the same plane 
and radiating at equal intervals from a com- 
mon center. The center of rotation is the center 
of the crank shaft. By placing all the cylin- 
ders in the same vertical plane a crank shaft of 
only one throw can be used, which means a 
centralizing of the forces exerted upon the 
crank shaft. With the throw placed vertically 
upward, the pistons will reach top center when 
their respective cylinders are directly above 
the single crank pin. If an explosion occurs 
within a cylinder when it is at top position the 
effect will be to increase the combustion space 
by revolving the cylinder farther away from 
the crank pin, which allows the piston to move 
away from the cylinder head. The force that 
revolves the cylinder is the force exerted by 
the piston upon the cylinder wall. 

Considering that the cylinders revolve in- 
stead of the crank shaft, it will at once be ap- 

1841 



\ 




\ 



\ 



\ 



/ 



\ 



V 

/ 



DIAGRAM TO ILLUSTRATE THE PRINCIPLE 
OF A ROTARY ENGINE 



ROTARY ENGINES 

parent that rotary engines will differ from 
fixed cylinder engines in the manner of sup- 
port, the way the cylinders are retained, the 
way the gasoline mixture is supplied to the 
cylinders, the way the valves are operated, and 
the way electricity is made to reach the spark 
plugs. 

Briefly, rotary engines are supported by two 
plates holding the rear end of the crank shaft 
which allows the engine to overhang its sup- 
port. The plate nearest the engines, which 
carries the magnetos and pumps is called the 
bearer plate. The rear one is the centralizing 
plate. The cylinders are retained by screwing 
them into the crank case and locking them 
with a lock nut, or by having the crank case 
made in two parts and clamping their flanged 
bases between the two halves of the crank case. 
In rotary engines the crank case is used to 
store the gas which is conducted to the cylinder 
either by means of ports in the cylinder wall or 
by individual manifolds from the crank case 
to the intake valves in the heads of the cylin- 
ders. The valves are operated by rods and 
rocker arms with sometimes an attempt to ful- 
crum the rocker arm at such a point that the 
centrifugal force acting upon the rod will be 

[85] 



ELEMENTS OF AVIATION ENGINES 

counterbalanced by that acting upon the 
rocker arm and the valve. Cam plates or cam 
packs revolving on the stationary crank shaft 
are used to operate the rods. Electricity is led 
to the spark plugs by means of bare brass wires 
drawn taut to segments imbedded in the re- 
volving thrust plate. A stationary brush 
protruding through the bearer plate has con- 
nection with the magneto and comes in contact 
with each segment as the engine revolves. 

The demand for increased power has led to 
the design of rotary engines with an additional 
bank of cylinders behind the first bank. The 
same general construction is used except that 
a crank shaft of two throws must be used with 
this type of engine. Those having one bank 
always have an odd number of cylinders while 
the two-bank engines have an even number. 
However, this even number is occasioned by 
an odd number of cylinders being on each of 
the two banks. The reason for an odd number 
of cylinders is to allow for an equal spacing of 
the power impulses. These engines, being four- 
stroke cycle engines, will have all cylinders 
function once in two revolutions. By using an 
odd number it is possible to fire alternate cy- 
linders as they come to top position, which 

[86] 



ROTARY ENGINES 

results in all cylinders having a chance to work 
once during two revolutions and still allow 
another cycle to be started without any inter- 
ruption. The cylinders are numbered in a 
clockwise direction viewed from the propeller 
end. 

While rotary engines are made almost en- 
tirely of steel, as a general rule they develop 
much more power for their weight than fixed- 
cylinder engines. This is due largely to the 
short crank shaft, the short crank case, and the 
fact that they are air cooled instead of water 
cooled. On account of the difficulty in supply- 
ing the revolving cylinders with gas, these en- 
gines use much more fuel proportionately than 
do fixed-cylinder engines. Due to their light 
weight they have become very popular for 
speed scout work, where brief but rapid flights 
are necessary. For great distances they are not 
looked upon with much favor because of the 
great quantity of fuel that must be carried to 
answer the engine's needs. For stunt flying 
rotary engines are admirably suited, on ac- 
count of their ability to work perfectly in any 
position. In any comparison of rotary and 
fixed-cylinder engines it must not be lost sight 
of that rotary engines, due to their radial form, 

[87] 



ELEMENTS OF AVIATION ENGINES 

offer the more head resistance. It is difficult to 
meet the demand for increased power with 
rotary type engines except in those cases where 
two-bank engines are used. An attempt to 
lengthen the stroke results in longer cylinders, 
which means more centrifugal force will be 
developed. The bore has limitations, because 
the sum of the diameters of the cylinders must 
be approximately the same as the circumfer- 
ence of the crank case. The compression can 
not be greatly increased, because the resulting 
increase in temperature is more than can be 
satisfactorily cared for by air cooling. 

The Gnome "monosoupape" is a well-known 
rotary engine which has attracted wide atten- 
tion. It is made in two sizes having one and 
two banks of cylinders. The nine-cylinder en- 
gine is the 100 H.P. Gnome, while the eighteen- 
cylinder one is the 180 H.P. Except for the 
number of cylinders the two engines are very 
similar. In entering into a description it is 
sufficient to take up the nine-cylinder engine. 

In any rotary engine great pains must be 
taken to prevent centrifugal force from carry- 
ing the cylinders away. In the nine-cylinder 
Gnome the crank case is split into two circular 
halves, and the cylinders are clamped between 

[88] 



ROTARY ENGINES 

the two parts. The cylinders, which are ma- 
chined from billets of steel, are drilled so that 
a ring of small ports is located near the base of 
each cylinder. These serve as an inlet valve, 
for, as a piston goes to bottom center, the ports 
are uncovered and direct connection is made 
between the interior of the crank case and the 
space beyond the piston head. The head of the 
cylinder is supplied with a large exhaust valve 
which gives rise to the name "monosoupape," 
French for single valve. 

The valve timing is novel inasmuch as it is a 
four-stroke cycle engine using a two-stroke 
cycle method of admitting the charge. The 
spark occurs 18° of the engine's rotation before 
top center. The power stroke is interrupted 
85° past top center by the opening of the ex- 
haust valve. This valve remains open 395° or 
120° past top center, allowing all the burned 
gas to be expelled and a supply of air to be 
drawn in during the 120° it remains open while 
the piston is going down. After the exhaust 
valve is closed the downward motion of the 
piston tends to create a partial vacuum within 
the cylinder so that when the intake ports are 
uncovered 20° before bottom center, a very 
rich mixture that is stored in the crank case 

[89] 



ELEMENTS OF AVIATION ENGINES 

will rush into the cylinder. The gas enters 
during the 40° that the ports are uncovered 
and mixes with the air that has been drawn in 
through the exhaust valve. A suitable mixture 
is thus formed which is compressed and ignited 
18° before top center. The reason for the early 
opening of the exhaust valve is to secure the 
same pressure within the cylinder as that with- 
in the crank case when the intake ports are 
uncovered on the power stroke. 

The rich mixture held in the crank case is 
formed by gasoline being sprayed from a noz- 
zle connected to the pipe that extends within 
the hollow crank shaft. The gasoline which is 
under a pressure of five pounds is led through 
a shut-off valve located in the cock pit for the 
pilot to control. Obviously the range of speed 
is greatly limited by this means of control, be- 
cause too lean a mixture is apt to result in 
back-firing and ruining the engine. A safer 
way to reduce the speed is to make the mixture 
too rich. 

Electricity is supplied to the spark plugs by 
a high-tension magneto located on the bearer 
plate, with a stationary brush bearing upon 
the segments that revolve at engine speed. 
The magneto must turn two and one-fourth 

[90] 



ROTARY ENGINES 

times engine speed since this is a four-stroke 
cycle engine of nine cylinders. Due to the fact 
that an ordinary magneto supplies but two 
sparks per revolution the magneto does not 
furnish a spark every time the brush is on a 
segment, but with a 2\i gearing it is capable of 
furnishing sparks for alternate segments. This 
is what is needed to obtain the right firing 
order. 

The connecting rods are made to work upon 
a hub that revolves upon the crank pin. One 
connecting rod called the master rod, is made 
integral with the hub to maintain its proper 
rate of rotation. The eight other connecting 
rods are pinned to the hub. The master con- 
necting rod prevents the lower ends of the con- 
necting rods from moving too far from their 
respective cylinders. In order that the hub 
may be mounted upon the crank pin it is 
necessary for the crank shaft to be made in 
two pieces. From this it follows that the crank 
shaft will be weakened so that the thrust of the 
propeller must be transmitted through the 
crank case to a thrust bearing at the rear of 
the engine. 

The pistons of the Gnome engine are made 
of cast iron with the piston bosses attached to 

[911 



ELEMENTS OF AVIATION ENGINES 

the concave piston head. The trailing edges of 
the skirts are cut away to prevent piston in- 
terference. The surface on the leading edge 
is not reduced as it will be remembered that it 
is the force of the piston against the cylinder 
wall that causes the engine to turn. On ac- 
count of the leading edge of a cylinder coming 
in contact with more air than the trailing edge, 
the expansion of a cylinder will be slightly 
irregular. This makes necessary a compression 
ring that will conform to the irregularity. A 
flexible L-shaped bronze ring known as an 
"obturator" is used. This ring is retained in a 
groove very near the piston head by means of 
a steel packing ring. The gap in the "obtura- 
tor" is placed on the leading edge where there 
is the least amount of clearance. The piston is 
also supplied with a cast-iron oiling ring. 

The exhaust valves are operated by rocker 
arms and push rods, which, with the tappets, 
radiate spirally from the cam pack located at 
the propeller end of the engine. The nine cams 
on the cam pack are designed with 197^° 
faces on account of the exhaust valve being 
held open for 395°. The cam pack is made to 
turn on the stationary crank shaft at half 
engine speed by a system of six planetary 

[92] 



ROTARY ENGINES 

gears. A thirty-tooth gear held rigidly upon 
the crank shaft meshes with two thirty-tooth 
gears pinned to the crank case. Each of the 
revolving thirty-tooth gears has a twenty- 
tooth gear secured rigidly to it, and it is the 
twenty-tooth gears that mesh with one having 
forty teeth attached to the cam pack. The re- 
duction of two to one is thus secured. 

Castor oil is delivered to two tubes within 
the crank shaft by a double plunger pump 
located upon the bearer plate. Pulsators are 
used to indicate the operations of the pumps. 
The oil from one tube goes to lubricate the 
front and rear bearings and the cam pack. 
The oil from the second tube is used to oil the 
connecting rod assembly and wrist pins. Spray 
from the connecting rod assembly comes in 
contact with the cylinder walls. Centrifugal 
force which carries the oil out the exhaust 
valves prevents the use of a circulating sys- 
tem. Due to oil being carried toward the 
cylinder head, it is impractical to place the 
spark plugs in the head. They are placed on 
the leading edge where they are less likely to 
become fouled. 

The Le Rhone engine with its threaded 
cylinders and peculiar valve operation has 

[93] 



ELEMENTS OF AVIATION ENGINES 
attracted a great deal of attention. Gas is sup- 
plied to the crank case by a crude carburetor 
attached to the rear end of the hollow crank 
shaft. A throttle and metering pin are used in 
the carburetor allowing a slightly wider range 
of speed than can be obtained with the Gnome. 
The inlet valve being located in the cylinder 
head, a separate manifold is used to conduct 
the gas from the crank case to each cylinder. 

The cylinders are of steel, and, being threaded 
at the base, they are retained by being screwed 
into the crank case and locked with a large 
lock nut. This design permits the compression 
to be changed by screwing the cylinders in or 
out as the case may be. A cast-iron liner 
shrunk into each cylinder does much to pre- 
vent irregular expansion from interfering with 
the piston rings. No ''obturator" is used on 
the Le Rhone. 

The inlet and the exhaust valves located in 
the cylinder head are operated by a single rod 
for each cylinder. A rocker arm is attached to 
each end of the rod. The base rocker arm is 
fulcrumed to the crank case with both ends 
supplied with rollers, each bearing upon a 
separate cam plate. These cam plates have 
five cams upon each one and are so constructed 

[94 1 



ROTARY ENGINES 

that when one end of the rocker arm is forced 
up by one plate the other end sinks into a de- 
pression on the second plate. In this way the 
rod is used both as a push rod to open the ex- 
haust valve and a pull rod to open the inlet 
valve. Since each cam plate has five cams they 
are revolved at nine-tenths engine speed. This 
rate is necessary because the valves must open 
nine times in two revolutions of the engine, 
and in two revolutions of the cam pack ten 
cams come into position. 

The connecting rods are designed with shoes 
at their large ends. The hub on the crank pin 
is made up of two discs each having three 
grooves to receive the connecting rod shoes. 
The discs are clamped together and hold the 
connecting rods between them. Each groove 
holds three shoes, and being a nine-cylinder 
engine, it follows that the connecting rods will 
be of three lengths. With this construction no 
master rod is necessary. 



[95] 



CHAPTER X 
THE LIBERTY MOTOR 

The liberty motor, which represents the 
latest development in aviation engines, 
is not known in detail by many at present. 
Due to the discretion of the War Department, 
little if anything could be learned regarding it 
during the time that the first engines were 
being built and tested. Now that its success 
is assured, the Committee on Public Informa- 
tion has given the writer permission to print 
the facts set forth in a recent number of "The 
Official Bulletin." The following paragraphs 
are authorized by the War Department and 
the Committee on Public Information: 

"The designs of the parts of the Liberty 
engine were based on the following: 

"Cylinders. — The designers of the cylinders 
for the Liberty engine followed the practice 
used in the German Mercedes, English Rolls 
Royce, French Lorraine, Dietrich, and Italian 
Isotta Fraschini before the war and during the 
war. The cylinders are made of steel inner 
shells surrounded by pressed-steel water jack- 
ets. The Packard Co. by long experiment had 

[96] 



THE LIBERTY MOTOR 

developed a method of applying these steel 
water jackets. 

"The valve cages are drop forgings welded 
into the cylinder head. The principal depar- 
ture from European practices is in the location 
of the holding-down flange, which is several 
inches above the mouth of the cylinder, and 
the unique method of manufacture evolved by 
the Ford Co. The output is now approxi- 
mately 1,700 cylinder forgings per day. 

"Cam shaft and valve mechanism above 
cylinder heads. — The design of the above is 
based on the Mercedes, but was improved 
for automatic lubrication without wasting oil 
by the Packard Motor Car Co. 

"Cam-shaft drive. — The cam-shaft drive 
was copied almost entirely from the Hall-Scott 
motor; in fact, several of the gears used in the 
first sample engines were supplied by the Hall- 
Scott Motor Car Co. This type of drive is used 
by Mercedes, Hispano-Suiza, and others. 

"Angle between cylinders. — In the Liberty 
the included angle between the cylinders is 
45°; in all other existing twelve-cylinder en- 
gines it is 60°. This feature is new with the 
Liberty engine, and was adopted for the pur- 
pose of bringing each row of cylinders nearer 

[97] 



ELEMENTS OF AVIATION ENGINES 

the vertical and closer together, so as to save 
width and head resistance. By the narrow 
angle greater strength is given to the crank 
case and vibration is reduced. 

"Electric generator and ignition. — A Delco 
ignition system is used. It was especially de- 
signed for the Liberty engine to save weight 
and to meet the special conditions due to firing 
twelve cylinders with an included angle of 45°. 

"Pistons. — The pistons of the Liberty en- 
gine are of Hall-Scott design. 

"Connecting rods. — Forked or straddle- type 
connecting rods, first used on the French De 
Dion car and on the Cadillac motor car in this 
country, are used. 

"Crank shaft. — Crank shaft design followed 
the standard twelve-cylinder practice, except 
as to oiling. Crank case follows standard prac- 
tice. The 45° angle and the flange location on 
the cylinders made possible a very strong box 
section. 

"Lubrication. — The first system of lubrica- 
tion followed the German practice of using one 
pump to keep the crank case empty, deliver- 
ing into an outside reservoir, and another 
pump to force oil under pressure to the main 
crank-shaft bearings. This lubrication system 

[98] 



THE LIBERTY MOTOR 

also followed the German practice in allowing 
the overflow in the main bearings to travel out 
the face of the crank cheeks to a scupper which 
collected this excess for crank-pin lubrication. 
This is very economical in the use of oil and is 
still the standard German practice. 

"The present system is similar to the first 
practice, except that the oil, while under pres- 
sure, is not only fed to main bearings but 
through holes inside of crank cheeks to crank 
pins, instead of feeding these crank pins 
through scuppers. The difference between 
the two oiling systems consists of carrying oil 
for the crank pins through a hole inside the 
crank cheek instead of up the outside face of 
the crank cheek. 

"Propeller hub. — The Hall-Scott propeller- 
hub design was adapted to the power of the 
Liberty engine. 

"Water pump. — The Packard type of water 
pump was adapted to the Liberty. 

"Carburetor. — A carburetor was developed 
by the Zenith Co. for the Liberty engine. 

"Bore and stroke. — The bore and stroke of 
the Liberty engine is 5 by 7 inches, the same 
as the Hall-Scott A-5 and A-7 engines, and as 
in the Hall-Scott 12-cylinder engine. 

[99] 



ELEMENTS OF AVIATION ENGINES 

"Remarks. — The idea of developing Liberty 
engines of 4, 6, 8, and 12 cylinders with the 
above characteristics was first thought of 
about May 25, 1917. The idea was developed 
in conference with representatives of the Brit- 
ish and French missions, May 28 to June 1, 
and was submitted in the form of sketches at 
a joint meeting of the Aircraft (Production) 
Board and the Joint Army and Navy Tech- 
nical Board, June 4. The first sample was an 
eight-cylinder model, delivered to the Bureau 
of Standards July 3, 1917. The eight-cylinder 
model, however, was never put into produc- 
tion, as advices from France indicated that 
demands for increased power would make the 
eight-cylinder model obsolete before it could 
be produced. 

"WORK ON THE 12-CYLINDER EN- 
GINE. 

"Work was then concentrated on the 12- 
cylinder engine, and one of the experimental 
engines passed the 50-hour test August 25, 
1917. 

"After the preliminary drawings were made, 
engineers from the leading engine builders 
were brought to the Bureau of Standards, 
where they inspected the new designs and 

[100] 



THE LIBERTY MOTOR 

made suggestions, most of which were in- 
corporated in the final design. At the same 
time expert production men were making sug- 
gestions that would facilitate production. 

"The Liberty 12-cylinder engine passed the 
50-hour test, showing as the official report of 
August 25, 1917, records 'that the fundamen- 
tal construction is such that very satisfactory 
service with a long life and high order of effi- 
ciency will be given by this power plant, and 
that the design has passed from the experi- 
mental stage into the field of proven engines'. 

"An engine committee was organized in- 
formally, consisting of the engineers and pro- 
duction managers of the Packard, Ford, 
Cadillac, Lincoln, Marmon, and Trego com- 
panies. This committee met at frequent 
intervals, and it is to this group of men that 
the final development of the Liberty engine 
is largely due." 



[101] 



INDEX 



INDEX 

PAGES 

Altitude Compensation 56 

Angle Between Cylinders 97 

Auxiliary Air Valves 56 

Battery Ignition System 68 

Bearing Liners 31 

Berling Magneto 64 

Bore 20 

Bosch Magneto 64 

Breathers 25 

Cam Shafts 36 

Castor Oil 74 

Centrifugal Pump 82 

Clearance of Pistons 27 

Clearance of Valve Stems 44 

Connecting Rods 29 

Crank Shafts 30 

Cycle 8 

Cylinders 22 

Cylinder Numbering 33 

Dixie Magneto 64 

Exhaust Valves 39 

Fire Point of Lubricating Oils 73 

Firing Orders 33 

Flash Point of Lubricating Oils .... 73 

Four-stroke Cycle Engines 9 

Gear Oil Pump 75 

Gnome Engine 88 

Horse-power 16 

[105] 



INDEX( Continued) 

PAGES 

Inductor Type Magneto 59 

Inlet Valves 40 

Le Rhone Engine 93 

Metering Pins 51 

Normal and Anti-normal Engines .... 20 

Oil-cooling Devices 78 

Pistons 25 

Piston Displacement 21 

Piston Rings 27 

Primary Current 60 

Propeller Speed 19 

Proportion of Gasoline to Air 47 

Secondary Current 62 

Shuttle Type Magneto 59 

Speed Compensation 50 

Stroke 10 

Thermosyphon Cooling System 81 

Thrust Bearings 35 

Two-stroke Cycle Engines 7 

Valve Grinding 40 

Valve Timing 43 

Vane Oil Pump 76 

Vibration 13 

Viscosity of Lubricating Oils 73 

Wrist Pins 28 

Zenith Carburetor 52 



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